Split Toning
Applying one colour tint to the highlights and a different one to the shadows.
Measured in
hue, saturation
In depth
The technique descends from darkroom toning, where selenium or sepia altered the print's colour in specific density ranges. Digitally it is a compact grading tool: warm highlights with cool shadows create depth and separation, and it is one of the few adjustments that works as well on a monochrome conversion as on a colour file.
Examples in practice
Warm amber in the highlights and cool blue in the shadows gives a black and white conversion a subtle sense of depth.