In a cellular system a base station has only a limited coverage area. Hence it is possible for a moving subscriber to get out of reach of a base station while making a call.
The process by which a mobile telephone call is transferred from one base station to another as the subscriber passes the boundary of a cell is called a handover.
In a CDMA network it is possible to switch from one base station to another without the need to switch the frequency. This is called a soft handover. If the connection has to be broken en set up on another frequency, it is called a hard handover.
In American-English a handover is called a hand-off.