The term comes from the old days of AM radio. A high frequency carrier wave was Amplitude Modulated with a low frequency audio signal. The two waves beat together, the audio signal being both added to and subtracted from the carrier wave. So if the carrier wave was one megahertz, which I still prefer to call cycles per second, and the audio signal was the note Middle C, which in those days was exactly 256 cycles per second, the result was a set of three high frequency signals, one at 999,744 Hz, one at 1,000,000 Hz, and one at 1,000,256 Hz.
In fact the audio was never a single frequency but ranged over a band of frequencies. That band of frequencies could include any in the allotted bandwidth. If the audio signals ranged from zero to 3,000 Hz, then the band of frequencies would be from 997,000 Hz to 1,003,000. That made for a bandwidth of 6,000 Hz. Anything outside of the 997,000 to 1,003,000 band could be allocated to another radio station.
Later people realized that since the lower half of the band was an inverted duplicate of the upper half, with no added information, there was no need to transmit it at all. Thus was born SSB, Single Side Band radio. Note that modern AM radio is just like the old kind, it transmits both sidebands. For even greater efficiency the central carrier frequency does not have to be transmitted at all, since it is devoid of information, but it is not easily suppressed.
To understand why modern computer people still use the term bandwidth, you need to understand that transmitting a binary signal, a series of 0 and 1 amplitudes still requires as much of a frequency spread as done with audio signals. The old telephone modems turned the string of bits into an audio signal using something like an earphone. This was presented as a sound to the mouthpiece of the telephone handset. Because the bandwidth of the telephone system was low, the number of bits per second which could be transmitted was low. Some telephone modems of this kind could only carry 110 bits per second. Things have changed! I will be uploading this post to the Internet over broadband cable at 100,000,000 bits per second.
What is the bandwidth of human communication? That’s hard to say. Speech is intelligible over a 2,000 bandwidth channel, perhaps even lower. But a lot of information is carried in tone and inflection, which comes across poorly over such a channel. Speech is a poor medium of communication by itself, though. Humans have evolved to transmit a lot of information through facial expressions and what is called ‘body language’. A person speaking to you in person with face and body clearly seen transmits a lot more information. Their bandwidth is higher. I don’t know how high.
The goal expressed on this website and on my Social Network Optimization website is to maximize overall human bandwidth, throughout the social network.