Bandwidth

Bandwidth is the capacity for traffic flowing in and out of your server — how much data can move through the connection at any given moment.

A site heavy with large images and video uses far more bandwidth than a plain text page. Some hosting plans cap or meter bandwidth, charging more when you exceed an allotment, so a sudden surge of popularity can carry a surprise bill. It's closely tied to scaling: handling more visitors usually means moving more data, and the bandwidth has to be there to carry it.