Affordance

The word comes from perception psychology, where James Gibson used it for the action possibilities an environment offers a creature: a branch affords grasping to a hand of the right size, whether or not anybody notices it. Don Norman brought it into design and, by his own account, muddled it, because designers started using affordance to mean the visual hint rather than the possibility. He proposed the fix himself: the possibility is the affordance, the mark that advertises it is a signifier, and a screen has almost no real affordances at all. A flat panel of glass affords touching, everywhere, equally. Everything else is drawn.

That is why the term is worth keeping precise. When someone says a button has good affordance they usually mean it is signified well: it has an edge, a fill, a shadow, a label in the imperative, a pointer that changes over it, a press state that answers. When they say a control has no affordance they mean the possibility is real but invisible, which is the more common failure. Flat design spent several years discovering this, and the lesson that stuck is that borders and fills were doing work nobody had costed.

Three failure modes are worth naming. A hidden affordance is real but unmarked, and only the people who already know it is there ever find it, which is what a swipe on a row or a long press without a menu ends up being. A false affordance is marked but not real: text with a hover underline that goes nowhere, an image that looks like a play button, a disabled control that looks pressable. And an affordance that only appears on hover is both, because half the readers have no pointer to reveal it with.

Conventions carry most of the load, and they are cheap. Underlined coloured text goes somewhere. A raised shape with a verb on it does something. A grip of dots says pick this up here. A shadow at the edge of a list says the content continues. None of these is a law of physics, they are learned, and the value of using them the way everyone else does is that a reader arrives already knowing what your interface allows.