Velocity is the rate at which team progresses Sprint by Sprint.
Velocity is a measure of the amount of work a Team can tackle during a single Sprint and is the key metric in Scrum. Velocity is calculated at the end of the Sprint by totalling the Points for all fully completed User Stories.
Velocity is an extremely simple, powerful method for accurately measuring the rate at which scrum development teams consistently deliver business value. It is an indication of the average amount of Product Backlog turned into an Increment of product during a Sprint by a Scrum Team, tracked by the Development Team for use within the Scrum Team.