Boom is defined in the Concise Oxford English Dictionary as “a movable arm carrying a microphone or film camera” and in Collins Concise as “a pole carrying an overhead microphone and projected over a film or television set“.
In sound studios a boom is either an arm fixed to a floor-mounted microphone stand or a combined arm and stand, often a tripod base with a telescopic arm extended by a winding handle. Larger wheeled versions are used in television and film studios, usually with a manoeuverable microphone mounting for an operator to “aim” the microphone (Fisher boom).
For mobile use (Location Recording) sound recordists use lightweight booms (or poles!), often confusingly referred to as “boom poles”. For even more confusion see Cue.