The amount of traffic that is allowed to occur between a site/server and the rest of the internet. In other words, bandwidth is the amount of traffic to allow transferring data between your site, your mail server and the Internet.
Bandwidth is measured in bits (a single 0 or 1). Bits are grouped in bytes which form words, text, and other information that is transferred over the Internet.
A note about discussion lists: Bandwidth for discussion lists is calculated by the total number of emails that are sent to the mailing list, as well as the reply emails that are sent by members of the discussion list to the discussion list. Any attachments that are sent with these emails are factored in to the total bandwidth. (list settings can be changed to deny attachments, to set daily message limits, to specify maximum message size, and more).