Ryan Singer wrote on Twitter:
Affordances are tricky to pin down because they’re nested. A hammer affords driving nails; its handle, swinging; it’s grip, grasping; etc.
I’ve always thought of affordances as elements, characteristics or properties that a designer introduces to explicitly communicate the possibility of an intended action.
For example, the lock-screen swipe control in iOS 6 doesn’t just provide for unlocking, it explicitly communicates through words and a particular visual control the right-swipe-to-unlock action. In iOS 7, though, we lose the visual control and are left only with the words, “Swipe to unlock”. (We are suggested a direction, though, by the right-moving light behind the words.)
According to Wikipedia, though, an affordance is any quality of an object which allows an individual to perform an action—such as the hammer qualities mentioned by Ryan.
But in the hammer example, the grip wasn’t added to communicate the action of gripping; rather it was added to provide for gripping. The handle wasn’t designed to communicate swinging; rather, it was designed to take advantage of the principle of leverage through swinging.
In my own thinking about affordances in the design of products, I think I’ll start differentiating between inherent affordances and explicit affordances; the latter intended to communicate, in addition to providing for, actions.