Affordance
A property of a form's shape that suggests, on its own, how it is meant to be used.
In practice
An affordance, as used here, is a property of an object's shape that signals how to use it without instructions. A flat metal plate on a door invites pushing; a vertical bar handle invites pulling; a round knob invites turning. When the shape suggests the wrong action, people fail, as with a door that has a pull handle but must be pushed.
Often confused with signifier
A signifier is an added cue, such as a push label or an arrow; an affordance, as used here, is suggested by the object's own shape.
Try this
Find three doors nearby and decide, before touching them, whether each must be pushed or pulled; then check.