= Number of total story points / One iteration
Velocity is a measurement of how much the team gets done in an iteration ( called as Sprint in Scrum ). Velocity is what actually got done in the last iteration not what is planned.
In Scrum it is measure in Story points. Each feature in scrum is a story. A story has points. Points can be anything you come up with.
Examples are 1, 2, 4, 8 , 16
5, 10 15 and so on.
A story depending on its complexity is given certain story points. So if the team does 6 stories that are 8 story points that iteration , the teams velocity is 48 story points.