"According to Norman, affordance refers to “the perceived and actual properties of a thing”, primarily the properties that determine how the thing could be operated. Note that perceived affordance is not the same as actual affordance. A facsimile of a chair made of papier-mache has a perceived affordance for sitting, but it doesn’t actually afford sitting: it collapses under your weight. Conversely, a fire hydrant has no perceived affordance for sitting, since it lacks a flat, human-width horizontal surface, but it actually does afford sitting, albeit uncomfortably.
The parts of a user interface should agree in perceived and actual affordances."
(Taken form MIT Courseware)