An affordance is a quality of an object or environment that allows one to perform an action. For example, in the physical world, a light switch affords flicking in two possible directions, but only one based on its current state.
Affordance is a term that quite a lot has been written about. Both Wikipedia and Interaction-Design.org have thorough explanations of its origins and tell the history of its adoption by HCI and design. In brief, the psychologist James J. Gibson published The Theory of Affordances in 1977, which he further explored in his book The Ecological Approach to Visual Perception in 1979. His wife, Eleanor J. Gibson, continued studying affordances in her own book, An Ecological Approach to Perceptual Learning in Development.
Don Norman introduced the term to the field of Interaction Design in his book, The Design of Everyday Things. In the process, he inadvertently broadened the meaning by not making a clear enough distinction between real and perceived affordances. He later corrected this in an article titled Affordances and Design.
I should have used the term “perceived affordance,” for in design, we care much more about what the user perceives than what is actually true. What the designer cares about is whether the user perceives that some action is possible (or in the case of perceived non-affordances, not possible).
The correction came too late, however, and we now use the term loosely as a suggestion or invitation. For example, we might say that the hover highlighting on a button affords clicking. Technically, that is incorrect usage. The highlight indicates that the button is clickable, sure enough, but we understand that due to past experience. It is a perceived affordance.
In 1991, Bill Gaver’s paper, Technology Affordances, appeared in the Proceedings of CHI. Therein, he described false affordances: perceived affordances that do not have any real function, leading the actor to believe that an action is possible when it is not. For example, if I were to style a word in this sentence blue with an underline, you would expect that clicking on it would open another web page.
For further reading on the subject, besides the links above, I recommend Joel Spolsky’s Affordances and Metaphors, which has several illustrated examples, and Dan Lockton’s post, Un-hiding an affordance, on Design with Intent.