Affordances are the characteristics of an object or device that present its uses.
The sharp blade of a knife affords cutting. Its handle affords gripping. Paper and pencil afford the possibility of writing and drawing. A doorknob affords opening.
The perceived sharpness of the blade is the affordance. The apparent fitness of the handle for our hand is the affordance.
If we put a doorknob in the middle of a wall, it suggests but does not (likely) afford opening the wall.
If we take away the blue color and underline from a hyperlink, it no longer appears to afford a jump in hypertext
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Some writers suggest we still use affordance for merely perceived or apparent uses, even when these are not really available (afforded).
Don Norman defined an affordance as "the perceived and actual properties of the thing, primarily those fundamental properties that determine just how the thing could possibly be used." Alan Cooper
? refined this definition by omitting "and actual", identifying perception as the crux of the problem. The whole point is that, through a combination of instinct and experience, we expect objects to behave in a certain way based on their appearance and we have very strong expectations if we've used an identical object before. If a designer violates one of these expectations, their user will suffer frustration every time they use the interface.
R. Howard UI GlossaryReferences:
Don Norman "The Design of Everyday Things"
Alan Cooper "About Face 2.0"