I learnt that bandwidth is the maximum amount of data that can be transferred over a network connection in a given period of time (so as I can understand It's the maximum speed this network connection could reach) but I see people calculating the bandwidth for websites like this (before purchasing a host): Required bandwidth = [Average website page size x Average page views x Average daily visitors x 30 x Redundancy] which seems no thing like the previous definition, so can somebody explain the difference for me?
Sometimes hosting companies use the term to refer to the maximum amount of data that the hosting account is allowed to transfer in and out of the server in a given period of time (often a month). If the account exceeds the limit they may suspend the account until the end of the period, or ask you to pay more.
I'd agree that isn't quite the standard meaning of bandwidth (although it's also not completely different either) but it would explain the nature of that calculation you quoted.