Bandwidth is the amount of information downloadable from the server over a prescribed period of time. In essence, it is the rate [data/time], but the time in this case is not seconds but rather a month or a week. So this rate is not like 56K or broadband, etc., which are also bandwidth but are measured per second.
What is commonly called "bandwidth" (both on this site and widely in the web hosting industry) is technically data traffic or web traffic. The technically correct meaning of the term "bandwidth" is data rate, or the maximum capacity of a data channel (for example an ADSL connection).