The Illinois Natural Areas Inventory defines natural areas as 
tracts of land or water that contain relatively undisturbed natural communities containing plants and animals that were present prior to European settlement.
These areas must also have the potential for protection and management. Natural areas are further categorized based on their attributes. These include 
  • whether or not endangered or relict species are part of the community
  • if there are outstanding geological or unique features
  • if the area is a nature preserve or land that is used for educational or scientific studies
  • if the area has an outstanding aquatic feature.
The quality of a natural area is determined by assessing the degree of disturbance to the natural community. Grades A through E are assigned to areas, based on their degree of disturbance. Grade A natural areas are relatively undisturbed. A grade A forest would be one that is ungrazed or old-growth, as opposed to second growth.

Grade B forests include those that are lightly disturbed, or have been minimally disturbed in the past. Forests that were selectively logged in the past might qualify as grade B natural areas. Grades C through E might be applied to second growth forests, with the degree of disturbance increasing with the increase in letter grade.