In astrodynamics a Δv or delta-v (literally "change in velocity") is a scalar which takes units of speed. It is a measure of the amount of "effort" that is needed to change from one trajectory to another by making an orbital maneuver.
Delta-v is produced by the use of propellant by reaction engines to produce a thrust that accelerates the vehicle.
Read more about Delta-v: Definition, Orbital Maneuvers, Producing Delta-v, Multiple Maneuvers, Delta-v Budgets