The best man at a wedding is the man who assists the bridegroom. The bridegroom's chief attendant at a wedding. the man who helps the bridegroom at a wedding ceremony
(film jargon) The first assistant to either the key grip (in charge of camera placement and movement) or the gaffer (in charge of lighting and electrics)
The assistant gaffer or the assistant to the key grip They are responsible for the routing and coiling of power cables necessary to run the lights for a shot
(Film) The person who helps the gaffer (= the person responsible for putting in and maintaining the electric wires) when a film or television programme is being made