Airtime

Actual time spent using a wireless phone. Most carriers bill customers based on how many minutes of air time they use each month. (Cingular)

The time tracked by wireless service providers to determine billing charges. Usage includes sending or receiving calls and other wireless transmission such as faxes, email, or data files. Some service providers may charge for a whole minute if only part of a minute is used. (Nokia)

The amount of time you spend using a cellular system, between the time you successfully place a call and the termination of that call. (Motorola)