measure of amount of effort to change trajectory

Delta-v, or Δv, (literally, the change in speed) in astrodynamics, (how spacecraft move in space) is a measure of the amount of effort (pushing power) needed to change a craft's trajectory (how it's going to move from now on) in space. It is measured by a unit of speed.

Abbreviations key:Escape Orbit (C3), Geosynchronous orbit (GEO), Geostationary transfer orbit (GTO), Earth–Moon L5 Lagrangian point (L5), Low Earth orbit (LEO) Graph showing the approximate required Delta-V for Mars, the Moon, and Earth, both ways. The red arrows show the directions in which it is possible to slow a craft down using the atmosphere of what it is going toward.

Most of the time, a propellant burned by an engine provides the needed thrust.

Delta-v is calculated in meters per second of thrust, or m/s.

For example, to reach Low Earth orbit, (LEO) a spacecraft generally needs about 9,300 to 10,000 m/s of thrust.