The perceivable property of an element that indicates how it can be used: the shading that says “press me,” the handle that says “drag me.”
The card is clickable but nothing affords it: no cursor change, no elevation, no hover response.
Strictly, an affordance is the action possibility itself; the visible hint is a signifier. Don Norman spent a career defending the distinction, and designers ignore it daily. The working rule survives the pedantry: every interaction needs a perceivable invitation. Remove it and you’ve created an affordance gap.