Affordance

The psychologist James J. Gibson introduced affordance in 1979 to describe the action possibilities an environment offers an animal, whether or not they are perceived. A surface affords walking on because of its physical properties, not because it looks walkable.

Don Norman brought the word into design in 1988 in The Psychology of Everyday Things, and in doing so shifted its meaning toward what the user can see is possible. He later judged that shift a mistake and separated the two ideas: the affordance is the actual possibility, and the signifier is the perceivable cue that advertises it.

Why the distinction is useful

A door affords pushing whether or not it says so. A flat plate on the push side is the signifier. Norman’s famous example is the door that affords both pushing and pulling but signifies neither, which is why it is now widely called a Norman door.

Screens have no physical affordances at all. Everything is signified: a button looks pressable because of shading, a border, a shape convention or its position. This is why flat design taken to its extreme causes trouble. Removing every shadow and border removes the signifiers, and users are left guessing what is clickable.

In practice

The test is whether someone can predict what will happen before they touch it. If a control needs a tooltip to explain that it is a control, the signifier is doing too little work.