Feathering
Aiming a light so the subject is lit by the softer edge of its beam rather than its centre.
In depth
The centre of any modifier is its brightest and most direct region; the edge is dimmer and, because light arrives there from a wider range of angles within the box, gentler. Feathering therefore softens the light and controls spill onto the background without moving the stand or changing the modifier.
Examples in practice
Turning a softbox so its axis points just past the subject's nose lets the far cheek fall off gently instead of flattening.