Key light
The main light on a subject, responsible for the picture's direction, shape, and basic exposure.
See it
What it is
The key light is the main light making the decisions. Its position sets which planes of the subject are bright, where the shadows fall, and usually the working exposure. It does not have to be the highest-output source in the setup, or even an artificial lamp. A window can be the key if it is the source giving the subject its shape.
Reach for a single clear key when the image needs readable direction. Move it near the camera for open, direct faces; take it around the side for texture and drama; raise it for the familiar shadow under the nose and chin. Rembrandt lighting is one named arrangement: a high, side key leaves a small triangle of light on the far cheek.
Gotcha: key means primary, not automatically hard, white, or powerful. A huge softbox can be a soft key and a desk lamp can be a dim key. Add several sources from competing directions at similar strength and the face picks up crossed shadows, so decide which light owns the shape before adding the rest.
Ask AI for it
Light the portrait with one clear key using the Rembrandt lighting technique: place a large softbox 45 degrees to camera left and slightly above eye level, exposing the near cheek cleanly and leaving a small triangle of light beneath the far eye. Keep the key one stop brighter than the ambient light, preserve a readable shadow side, and do not add a second competing key or crossed nose shadows.