Key Light
The dominant light in a setup, which establishes the direction of the shadows and the overall exposure.
Measured in
f-stops (EV)
In depth
Every other light in the scene is defined in relation to the key: fill reduces the shadows it casts, rim separates the subject from what is behind, background light controls what is beyond. Deciding the key's position, height, size and distance is therefore the whole lighting decision in miniature, and everything else is adjustment.
Examples in practice
A large softbox placed 45° to the left and slightly above eye level sets the key, metering f/8 at the subject's cheek.