Delta-vee (or Δv) is the measurement for change in velocity of a spaceship or other stellar object to escape gravitational forces. Usually the term is used synonymously for a ship's capability of changing its course in different maneuvers until all its power is depleted.
When the crew of the USS Enterprise-D tried to restore the orbit of Bre'el IV's moon, a delta-vee of four kilometers per second were needed. The Galaxy-class vessel was not able to produce that speed increase in time even with powering up the tractor beam and exceeding the recommended impulse engine's power limit by 47%. (TNG: "Deja Q")