Bald Knob – This rarely used term refers to a glacial deposit in the shape of an isolated hill. This hill is treeless (hence, it is 'bald') and surrounded by relatively flat land. In the absence of development, a bald knob supports prairie vegetation. With greater elevation, a bald knob can support rocky alpine vegetation, although this type of habitat is not found in Illinois.

Barrens – This ecological term is somewhat ambiguous in its meaning. As it is used here, a "barrens" refers to a relatively open upland area that is very rocky or sandy (similar to a glade, but less prairie-like). A barrens is capable of supporting a few stunted trees, small shrubs, and some ground vegetation. In the past,  a "barrens" often referred to any open area that had been burned-over, regardless of the characteristics of the soil. It was assumed that such burned-over areas had sterile soil, which was often not the case.

Beach – A low sandy area along a major lake or river. The lower area of a beach that is adjacent to a body of water supports very little vegetation as a result of frequent wave action, however the upper area of a beach can support small shrubs and various kinds of ground vegetation that are adapted to very sandy areas.

Bluff – This is a wooded hill. Bluffs often occur along major rivers. They tend to have smaller and less densely distributed trees than woodlands on lower ground. As a result, they often support more shrubs and ground vegetation.

Bog – This is a wetland that is supplied with water solely from rainfall, rather than through a river, spring, or seep. The soil of a bog tends to be dominated by sphagnum moss or peat and it is highly acidic. Because of the acidic water and low level of nutrients in the soil, bogs support a more restricted variety of vegetation. Open bogs support primarily ground vegetation consisting of grasses, sedges, forbs (flowering dicots), and low shrubs, while forested bogs support such trees as Tamarack (Larix decidua) and Black Spruce (Picea mariana). A forested bog that is dominated by Tamarack is referred to as a 'Tamarack Bog.' Bogs occur primarily in more northern areas where the climate is cool and moist.

Canyon – This is an unusually large ravine that has a relatively large flat bottom with steep rocky slopes on either side. The bottom of the canyon consists of a river and the surrounding flood zone area. Canyons often contain some trees, but they are stunted in size and more sparsely distributed than surrounding wooded areas. This is a rare ecological system in Illinois.

Cliff – A steep rocky slope, often facing a river valley or a canyon. In Illinois, cliffs are usually found in the southern part of the state, where they may consist of either limestone or sandstone. Some kinds of vegetation are often found at the top of cliffs, while other kinds of vegetation are found at the bottom of cliffs. Some small ferns have specialized in colonizing the crevices of cliffs; they often display a  preference for cliffs consisting of either sandstone (acidic substrate) or limestone (alkaline substrate).

Fen – This is a relatively open wetland that is supplied with water from an underwater spring or seep. The mineralized ground water of a fen can support a variety of ground vegetation, including some shrubs and small trees. Where Northern White Cedar (Thuja occidentalis) is abundant in this kind of wetland habitat, it is referred to as a 'White Cedar Fen.' The pH of the soil in a fen varies from mildly acidic to alkaline.

Flatwoods – A flatwoods occurs on a depression in an upland wooded area where the underlying soil contains a layer of clay, causing poor drainage. A flatwoods contains trees that tolerate considerable variation in moisture levels, from flooded conditions during the spring to droughty conditions during late summer.

Glade – This is a meadow that occurs on thin rocky soil in a hilly wooded area. Glades are normally located on top of a hill or mountain where a plateau exists. They are usually surrounded along their margins by trees and other woody vegetation (typically woodlands or savannas). The rocky soil is usually too thin to support trees, although some glades are maintained by occasional fires. The underlying bedrock of a glade can consist of limestone, sandstone, chert (quartz-laden rock), and other rocky material. Glades are found primarily in southern Illinois.

Gravel Bar – This is a low deposit of gravel along a major river. Gravel bars are sometimes created as a result of repeated flooding along major rivers.. They support primarily herbaceous ground vegetation that can colonize moist gravelly areas that are subjected to occasional disturbance.

Ledge – A ledge usually refers to a short rocky shelf along a steep vertical cliff. A ledge may be located at the top of a cliff, or it may be located at a lower level of the cliff. Cliffs develop ledges in response to past water erosion from flooded rivers. As a river sinks lower into the bedrock, it can create new ledges during floods. Different kinds of ground vegetation can be found along ledges, depending on whether it is an exposed ledge (facing the wind and sunlight), or a sheltered ledge (protected from wind and sunlight).

Marsh – This is a wetland with a small body of open water that is surrounded primarily by emergent aquatic vegetation and some ground vegetation, especially sedges, bulrushes, and cattails. Some wetland shrubs (e.g., willows) may colonize drier ground along the edge of a marsh. A marsh may be adjacent to a lake or slow-moving river, or it may be isolated from other wetlands. The ground soil of a marsh may be sandy or muddy and, unlike the ground soil of a bog, it is not strongly acidic.

Meadow – As used here, a  "meadow" refers to a sunny open area that is often surrounded by trees and other woody vegetation. The ground vegetation of such meadows consists of grasses, sedges, and forbs (flowering dicots). Some low shrubs and vines also occur in this ecological system. Sometimes a meadow occurs along a river in an area that is either too damp for most woody vegetation, or where beavers destroy most of the woody vegetation. A special kind of flood-prone meadow where sedges are dominant is called a "sedge meadow." Meadows tend to occur where wooded areas are the dominant ecological system, although some meadows are surrounded by either water or development.

Panne – This is a low flat sandy area that is prone to temporary flooding by the water of a lake. Over time, a panne accumulates minerals (including salt) that are suspended in the water as a result of repeated evaporation. In Illinois, this habitat is found primarily along Lake Michigan, where it is uncommon. Certain kinds of ground vegetation and semi-aquatic vegetation can be found in sandy pannes because of their need for minerals or their tolerance of saline conditions.

Parkland – When this term is used in an ecological context, it refers to a wooded area (usually somewhat open) that is mostly lacking in shrubs, saplings, and other low woody vegetation. A parkland can be the result of relatively frequent ground fires that spare fire-resistant canopy trees, or it may result from heavy grazing or mowing. The ground vegetation in a parkland consists primarily of such herbaceous plants as grasses, sedges, and forbs (flowering dicots).

Pioneer Cemetery – Some cemeteries in Illinois were established during pioneer days. Because of neglect, a small number of these cemeteries and areas adjacent to these cemeteries have native ground vegetation that has largely disappeared from more developed areas of the state. Typically, such pioneer cemeteries and adjacent areas are overgrown with prairie vegetation, although some of them also have savanna and woodland vegetation along their edges.

Powerline Clearances – Electric power companies often remove woody vegetation underneath powerlines. Such clearances create temporary meadows where various kinds of ground vegetation and small shrubs are dominant. When trees and other woody vegetation recolonize such areas, they are cut to ground level yet again, enabling the resurgence of ground vegetation. This cycle is repeatedly indefinitely. Powerline clearances reduce or remove trees and other woody vegetation from woodlands, overgrown fence rows, and other habitats. Telephone companies engage in similar activities to a lesser extent.

Prairie – An open treeless area consisting of grasses, sedges, and forbs (flowering dicots) as the dominant ground vegetation. Some sub-shrubs and vines also occur in this ecological system. Where prairies occur, they are usually the dominant ecological system. As a result, prairies tend to surround wooded areas, rather than the other way around. The moisture gradient of prairies can vary from wet to dry. Different kinds of prairie have been identified according to their soil type. In Illinois, this includes black soil prairie, sand prairie, dolomite prairie, and gravel prairie. In the past, prairies dominated the northern two-thirds of Illinois.

Ravine – This is a long deep depression that is surrounded on both sides by steep slopes. Many ravines occur in rocky wooded areas where the underlying bedrock has been exposed by water erosion. Either temporary or permanent streamlets are often present at the bottom of deep ravines.

Sand Bar – This is a low deposit of sand along a major river or lake. Sand bars are created as a result of repeated flooding or wave action. They support primarily herbaceous ground vegetation that can colonize moist sandy areas that are subjected to occasional disturbance.

Sand Dune – This is a hill consisting primarily of sand that is sometimes found along major rivers or lakes. Some inland sand dunes are relatively isolated from major bodies of water because the rivers or lakes that created them have disappeared from the vicinity a long time ago. Such extinct rivers or lakes can be dated to an earlier time period, often shortly after the last glaciation. Young sand dunes support very little vegetation, but over time sand dunes become more stabilized, when they can support some trees and other vegetation.

Savanna – This is a thinly wooded area that admits some sunlight between the trees. The trees in a savanna are more widely spaced apart than the trees of a woodland. Because of the greater availability of sunlight, savannas tend to support a greater variety of shrubs, vines, and ground vegetation than a dense woodland. A savanna can occur as the result of fire or other disturbance destroying some of the trees, or it may result from a natural thinning of the trees because of the characteristics of the soil (e.g., too sandy or rocky to support abundant trees). In Illinois, there are typical savannas with loamy soil, sandy savannas, and rocky upland savannas. Oaks are often the dominant trees in a savanna.

Seep – This is a wetland that is caused by ground water slowly oozing to the soil surface, creating an area that is more or less permanently damp and soggy. A seep may occur on relatively low flat ground, or it may occur along a hillside from the horizontal movement of ground water. At some seeps, shallow pools of water may accumulate. Seeps can occur in either open areas or wooded areas. In a wooded seep, the tree canopy tends to be less dense than in drier areas.

Slough – This is a river-like depression in a relatively open area that becomes temporarily flooded after a heavy rain. Afterwards, the slough dry up into puddles of water and exposed ground. The water of a slough migrates (usually rapidly) to lower ground in another area.

Swale – This is a wet pool-like depression in an open area that is prone to temporary flooding. The water of a swale is stagnant and doesn't migrate to other areas. When a swale occurs between adjacent sand dunes it is called an "interdunal swale." Many swales are found in prairies.

Swamp – This is a wetland that is dominated by flood-resistant trees. In southern areas, Bald Cypress (Taxodium distichum) and Water Tupelo (Nyssa aquatica) are common trees of swamps, while in more northern areas swamps are dominated by various deciduous trees. Swamps usually have standing water at certain times of year, although they are not usually permanently flooded in most places. Drier areas of swamps support a variety of understory and ground vegetation, while wetter areas support both aquatic and emergent-aquatic plants.

Thicket – This is an area that is dominated by shrubs and vines, rather than trees or low ground vegetation. A thicket may occur at the edge of a wetland, woodland, or prairie. Thickets may colonize burned-over woodlands or invade prairies in the absence of fire. They also occur in wetlands that are too soggy to support trees. When wetland areas are dominated by either willows (Salix spp.) or alders (Alnus spp.), they are referred to as 'Willow Thickets' and 'Alder Thickets.'

Vernal Pool – This is a pool of water in a wooded area. A vernal pool is usually filled with water after a period of rainy weather (especially during the spring), but it has a tendency to dry out in response to a protracted drought. Vernal pools often support shade-tolerant wetland vegetation along their margins and they provide breeding grounds for certain amphibians (e.g., frogs and salamanders).

Waste Ground – This refers to open ground in a developed area that is largely ignored or poorly maintained. It may refer to areas around junk yards, landfills, vacant lots, neglected areas along back alleys, rail yards, grounds surrounding abandoned buildings, and various edge habitats between developed sites.

Woodland – This is an area that is dominated by canopy trees. Underneath these canopy trees, there is usually other kinds of vegetation consisting of shade-tolerant understory trees, shrubs, woody vines, and herbaceous plants. However, some canopy trees may cast shade that is sufficiently dense to kill off understory and ground vegetation. Old-growth woodlands are dominated by large old trees that eventually topple and fall to the ground. Second-growth woodlands are dominated by younger trees that have not achieved their full size. Woodlands can be dominated by either deciduous trees or coniferous trees, although some woodlands are mixed (consisting of both deciduous and coniferous trees). Floodplain woodlands usually occur along the flood zone of rivers, although sometimes they are on low ground that is far removed from major bodies of water. Upland woodlands are more dry and better-drained, although they may contain small moist depressions.

Woodland Border – This refers to the edge of a woodland along an open area. This open area may consist of a prairie or meadow, a clearance along a road or some other developed area, or an open wetland.

Woodland Opening – This refers to a sunny opening in a woodland that is too small to be considered a meadow. Woodland openings are often caused by fallen trees.