Signifier
Also said as Visual language, Yellow paint, Environmental cue
Any perceivable hint that an action is possible here: paint on a ledge, a glowing crack, a handle on a door. Don Norman brought the word into design to separate the thing you can do from the sign that tells you so.
Without them, players must try everything or give up. With them, you can make huge spaces easy to read without a word of text. Be careful: once players learn that only painted ledges work, an unpainted one is a wall.
Say it
“Every climbable ledge gets a strip of worn yellow paint, the same across the game. Nothing else in the level may be that yellow.”
Where you’ve seen it
Mirror's Edge turns useful scenery red to show you what to run along and climb, which it calls Runner Vision. It can be turned down or off once you know the language.
Not to be confused with affordance
An affordance is what you can do with something. A signifier is what tells you. A ledge you can climb has the affordance whether you notice it or not; the paint on it is the signifier.