Butterfly lighting uses only two lights. The key light is placed directly in front of the subject, often above the camera or slightly to one side, and a bit higher than is common for a three-point lighting plan. The second light is a rim light.
Often a reflector is placed below the subject’s face to provide fill light and soften shadows.
This lighting may be recognized by the strong light falling on the forehead, the bridge of the nose, the upper cheeks, and by the distinct shadow below the nose that often looks rather like a butterfly and thus, provides the name for this lighting technique.
Butterfly lighting was a favourite of famed Hollywood portraitist George Hurrell, which is why this style of lighting is often called Paramount lighting.
From Wikipedia article which is released under the Creative Commons Attribution-Share-Alike License 3.0.