Hackney carriage or cab, means any such vehicle for the conveyance of passengers which plies for hire, whether drawn or propelled by animal or mechanical power, and which is not a stage carriage or a tram car. [See the London Cab Act, 1896, s. 3]
The phrase 'ply for hire' does not connote motion A Cab is not plying for hire when cruising whilst displaying a 'for hire' sign unless the driver stops when hailed, see Hunt v. Morgan, (1949) 1 KB 233: (1948) 2 All ER 1065 DC