When the transmitter is not modulated, or the amplitude of the modulating signal is zero, the frequency of the carrier is called its:

In a Frequency Modulation (FM) system, the carrier wave continuously shifts its frequency back and forth in response to an applied audio or data signal. This applied signal is the modulating signal. When no modulating signal is present (or its amplitude is zero), the carrier wave rests at its steady, unmodulated baseline frequency. This resting point is referred to as the centre frequency. Terms like 'frequency deviation' or 'frequency shift' describe the movement away from this baseline, while 'modulating frequency' refers to the audio signal itself, not the carrier.