"Bandwidth is the measure of the capacity of a communications channel.

Bandwidth (in terms of digital data) is the maximum amount of data that can travel along a communications channel in a given time."

The smallest form of digital data is the BIT and time interval is measured in Seconds

And so a sensible measure of the data capacity is "bits per second".

However, a channel having a capacity of only a few bits per second would extremely slow!

Think "Lighthouse". You are out at sea on a stormy night and you see a lighthouse signal - it flashes ON and then it is OFF for a while. This is digital data - the lighthouse signal takes a second or so to flash, so in this case the bandwidth would be about 1 bit per second.