Rim lighting
A bright edge tracing an object's silhouette that lifts it off the background — borrowed from the backlight of photography and film.
In a studio you put a lamp behind the subject so a thin band of light draws its outline; in real-time graphics the same thing is faked straight in the shader. You measure how edge-on the surface faces the camera — a term of the form one minus the dot product of normal and view direction — and brighten exactly those surfaces about to vanish around the rim. An exponent controls how narrow the band is.
The maths is the same as in the Fresnel effect, but here it is used for style rather than physical accuracy. The effect is cheap and readable, which makes it common in cel shading and anywhere a character has to stand out clearly against a busy background.