Backlighting
Placing the main light source behind the subject, so it faces the camera rather than illuminating the subject's front.
In depth
Backlight produces rim separation, glowing translucent edges on hair and leaves, and — if the front is not filled — silhouette. It also guarantees a metering problem, because the meter sees the bright source and underexposes the subject, and a flare problem, because the source is shining into the lens. Both are managed rather than avoided.
Examples in practice
A low sun behind a subject rims their hair with gold while a reflector in front keeps the face two stops brighter than silhouette.