A type of lighting position located on the side walls or in the box seat area of a theatre (hence the name "box boom). This offers the designer a lighting angle on an angle (similar to 45 degrees, but the actual angle depends on the architecture of the theatre itself).
http://www.mirvishproductions.com/My%20Files/Ed%20Mirvish/Ed%20Mirvish%20HL%20Box%20Boom.jpg
http://www.mirvishproductions.com/My%20Files/Ed%20Mirvish/Ed%20Mirvish%20HL%20Box%20Boom.jpg