The Collaborative International Dictionary of English v.0.48:

CB \CB\ n.
   same as citizens' band; that portion of the radio frequency
   spectrum allocated by the FCC for the use of individual
   citizens for short-distance personal or business use, from
   either fixed or mobile stations. Also used attributively, as

   CB radio. [acronym]

   Syn: Citizens' Band.
        [PJC]