Rim light
Also said as Rim lighting, Back light, Edge light, Fresnel highlight
A thin line of light along the edge of a shape, as if something bright stands behind it. In games it is often calculated: the more a surface turns away from you, the brighter it gets.
It pulls a dark character away from a dark background without changing the mood of the scene. It is one of the cheapest ways to protect a silhouette.
Say it
“Add a cool rim light to the player, fresnel power 3 at 60% strength. Keep it on in dark rooms, so the silhouette never melts into the background.”
Where you’ve seen it
Team Fortress 2 adds rim highlights to its characters. Valve's own commentary says they emphasise the outline, and that without them a character blends into the scenery.
Not to be confused with outline
An outline is a drawn line of constant colour. A rim light behaves like light: it reacts to the shape and the angle.