What is Baseband?

Legal Definition
Baseband is a signal that has a very narrow frequency range, i.e. a spectral magnitude that is nonzero only for frequencies in the vicinity of the origin (termed f = 0) and negligible elsewhere. In telecommunications and signal processing, baseband signals are transmitted without modulation, that is, without any shift in the range of frequencies of the signal, and are low-frequency—contained within the band of frequencies from close to 0 hertz up to a higher cut-off frequency or maximum bandwidth. Baseband can be synonymous with lowpass or non-modulated, and is differentiated from passband, bandpass, carrier-modulated, intermediate frequency, or radio frequency (RF).
-- Wikipedia
Legal Definition
When bandwidth is sent on a single signal. Only one channel is usable at a time. Its cheaper and simpler. Local area networks use this.