interaction
Signifier
The explicit cue that tells you how to act, layered on top of what's merely possible – the "push" plate on a door that affords both pushing and pulling.
Norman’s refinement of his own affordance idea: an affordance is what an object makes possible; a signifier is what communicates it. A door affords both push and pull – the flat plate signifies “push”. The distinction matters because most usability failures aren’t missing possibilities, they’re missing communication.
In interfaces, signifiers are the underline on a link, the dotted border under the glossary terms on this site, the placeholder text in an input, the grab-handle dots on a draggable row. When a user hovers somewhere and mutters “can I click this?”, a signifier is missing.