In astrodynamics, the term delta-v, literally "change in velocity" (see symbol delta), has a specific meaning: it is a scalar which takes units of speed that measures the amount of "effort" needed to carry out an orbital maneuver, i.e., to change from one trajectory to another. Delta = int_^ }, dt Where T is the instantaneous thrust m is the instantaneous mass In the absence of external forces, and when thrust is applied in a constant direction this simplifies to: = int_^ , dt = | _1 - _0 |