Baseband signals are the fundamental group of frequencies in an analog or digital waveform that may be transmitted along a pathway or processed by an electronic circuit. Baseband signals can be composed of a single frequency or group of frequencies or in the digital domain composed of a data stream sent over an unmultiplexed channel. Examples of an analog baseband signal may be audio or composite video. Examples of a digital baseband signal may be Ethernet signals operating over a Local Area Network (LAN).

Signals of a baseband nature often are modulated, or multiplexed, with other signals to form a composite signal. Sometimes, as in for example FM analog broadcasting, left and right channel audio signals are mixed to form a single channel (L+R) which is then modulated on the radio carrier but still considered the baseband of the composite FM signal. These same two channels are also subtracted (L-R) to form a difference channel which is modulated onto a subcarrier to minimize interference with the baseband. This modulated L-R or difference signal is considered a sideband. It may appear to be a conflicting definition, since sometimes a baseband signal is in itself a mixed signal. In reality, baseband is a term the meaning of which depends somewhat on the context of the statement. Another way of saying this is that mixing a baseband signal with a carrier signal normally results in a modulated signal which may or may not still be considered baseband, and that depends on what else is happening in the transmission envelope.