Baseband has several usages:
1) Describing a telecommunication system in which information is carried in digital form on a single unmultiplexed signal channel on the transmission medium. This usage pertains to a baseband network such as Ethernet and token ring local area networks.
2) Same as the above, but allowing that the information could also be carried in analog form.
3) Any frequency band on which information is superimposed, whether or not the band is multiplexed and information is sent on subbands. In this usage, there is sometimes the meaning that the frequency band is not shifted to some other frequency band but remains at its original place in the electromagnetic spectrum.