Affordances
"Affordances are potential complementary relationships between the organism and the environment. Information specifying affordances is directly available to the perceptual system in the ambient optical array."
Van Leeuwen, L., Smitsman, A., & van Leeuwen, C. (1994). Affordances, perceptual complexity, and the development of tool use. Journal of Experimental Psychology: Human Perception and Performance, 20, 174-191.
An affordance is an environmental property. This property's optimal value (relevant to an actor's performance of an action like climbing or sitting) is an "invariant proportion of some aspect of the actor's body scale."
Mark, L.S., & Vogele, D. (1987). A biodynamic basis for perceived categories of action: A study of sitting and stair climbing. Journal of Motor Behavior, 19, 367-384.
"Affordances describe the functional utility of particular objects within the performance context in reference to the performer's action."
Wu, C., Trombly, C.A., Lin, K., & Tickle-Degnen, L . (1998). Effects of object affordances on reaching performance in persons with and without cerebrovascular accident. American Journal of Occupational Therapy, 52, 447-56.