Baud rate is the number of signal units (symbol changes) transmitted per second over a communication channel. It measures the rate of change of the physical signal, not the number of bits transferred.

Baud rate equals bit rate only when each signal unit carries exactly 1 bit (binary signaling). In general:

Bit rate = Baud rate × log₂(L)

where L is the number of distinct signal levels used per symbol. With more levels per symbol (e.g. QAM), the bit rate can exceed the baud rate.