A telephone service that alerts someone using the phone to an incoming call and allows switching between calls.
A telephone service that allows a person already talking on the telephone to be alerted to an incoming call by a faint beep or click and to take that call by putting the first call on hold.
A voice telephone system (e.g., PBX or Centrex) feature that allows an incoming call to wait in queue if the target station is engaged. A call waiting signal alerts the user to the fact that a call is waiting. See also camp-on.