Affordance is a property of an object that naturally indicates how the object can be used.

Affordance is a desirable property of an object that naturally leads people to take the correct steps to accomplish their goals.

When used in context of digital design, it should be used (in the sense) of perceived affordance*1.

It is often confused with visual feedback, eg: All screens afford touching - only some detect the touch and are capable of responding. But the affordance of touchability is the same in all cases. Touch sensitive screens often make their affordance visibly perceivable by displaying a cursor under the pointing spot. The cursor is not an affordance; it is visual feedback.

1 - Don Norman : Affordances and Design