While understandable how one may get there, this definition does not make sense:
Band-pass signal/channel: A channel/signal whose bandwidth does not start from 0Hz.
While this may be true, it does not capture the real essence of band-pass. Thus, the definition of low-pass is similarly misguided.
Zero hertz is a degenerate case, because the concept of frequency is based on multiplicativity (division, technically, but that's just the inverse-multiplication function) and zero has a special meaning in multiplication. Any channel can exhibit a zero hertz signal, analog or digital, if you simply switch it off.
It would be better if you thought of all signals as having frequencies that are:
- non-zero, so perhaps infinitesimally low-frequency, but greater than zero, and,
- relative, such that one signal is of a lower, equal or higher frequency than another.
An ideal low-pass or high-pass filter would have a single cutoff. In the case of a low-pass, signals of frequencies lower than the cutoff are passed. Thus, low- (frequency) pass (not cut off but allowed to pass).
A band-pass filter passes a band, higher than a certain cutoff and lower than another cutoff. The frequency range between the cutoffs, that is passed, is the pass band, hence again the name.
There are many varieties of modulation but many useful ones have a net spectral effect of raising the frequency. A very common real-world use case is to modulate a low-frequency signal like human voice onto a high-frequency carrier like easily transmittable RF, or block coding for error correction.
Therefore very often a baseband signal will have a lower frequency than the desired channel frequency. Since the baseband is generally available on the transmission end of a channel and it is lower frequency, placing a low-pass filter between baseband input and modulation with a cutoff between the expected channel frequencies can often have the net effect of increasing the SNR.
As stated in some other answers, there are cases where it is useful to high-pass-filter your baseband as well. In some of these cases and depending on your application, it may be useful to band-pass-filter this baseband before modulation.