One of the transmitters on a satellite.
A transponder transmits on a particular frequency to which a set-top box must tune to receive a signal. Typically each transponder can carry multiple compressed digital television channels.
In some cases an interactive service, or an element of the service, may be carried on a different transponder, or even a different satellite. This may require the set-top box to re-tune to acquire the new signal, a process sometimes referred to as a ‘transponder hop’. This is typically masked by the presentation of an appropriate graphic to cover the interruption that may last several seconds.