What is meant by the term “baseband” in radio communications?

The term 'baseband' refers to the raw, unmodulated message signal before it is impressed onto a higher-frequency radio carrier. It represents the original frequency range occupied by the information you want to transmit. For example, in an SSB voice transmission, the baseband signal is the human speech audio ranging from roughly 300 to 3000 Hz. In digital communications, it represents the raw data bit stream before the modulation stage converts it into an RF signal. It is easy to confuse this term with RF concepts, but baseband has nothing to do with the lowest frequency band a transceiver can tune, nor is it the unmodulated bandwidth of the transmitter. It strictly describes the input message signal itself.