Attention, memory and perception
Signifiers
Also called Affordances
Visual cues that tell people what they can do with something, such as a raised button, an underlined link or a handle you can drag.
An affordance is what something lets you do; a signifier is what tells you so. A flat rectangle might be clickable, but nothing says it is.
Make interactive things look interactive and static things look static. Consistent signifiers across a product let people learn them once.
In practice
A card that lifts slightly and shows a pointer on hover tells people it opens; a plain one with the same behaviour leaves them guessing.
Common mistake
False signifiers: underlined text that isn't a link, or a greyed-out look on a button that works.
Where it comes from
Psychologist James Gibson coined affordance in 1977. Don Norman brought it to design in The Design of Everyday Things (1988) and added signifiers in the 2013 edition.