In mobile phone systems, a handoff is the process of transferring a phone call in progress from one base station (tower) to another base station, without interruption of the call.
"Soft" handoffs are unique to systems based on CDMA technology. During a soft handoff, a phone may be actively connected to multiple base stations simultaneously, possibly for a considerable length of time.
This contrasts with non-CDMA systems, (such as GSM,) where a phone may only be actively connected to one tower at a time. On such systems, handoffs are "hard", meaning they happen at a specific moment when the call is shifted completely from one base station to another.