Silhouette
Also said as Silhouette test, Outline read
The outline of a character or object, filled in solid. The silhouette test asks whether you can still tell who it is with all colour and interior detail gone.
In a real fight, characters are small, far away and moving, and the outline is all you get. If two characters share a shape, players will mistake one for the other.
Say it
“Fill all four heroes in black and put them side by side. If you cannot name each one in under a second, change the outline: bigger shoulders, a hat, a long weapon. Do not fix it with colour.”
Where you’ve seen it
Valve's 2007 paper Illustrative Rendering in Team Fortress 2 says the nine class silhouettes were carefully designed to be very distinct, because telling classes apart is the core of the game.
Not to be confused with rim light
A silhouette is the shape itself. Rim light is one way of separating that shape from a dark background.