Delta-v (written as dv or delta-v) is the scalar measure of the change in velocity a spacecraft must achieve to perform a given orbital maneuver, expressed in meters per second (m/s). It is the single most important parameter in mission design because it directly determines the amount of propellant required, which in turn drives vehicle sizing, cost, and feasibility.
Getting from Earth's surface to LEO requires roughly 9.4 km/s of delta-v (including gravity and drag losses). Transferring from LEO to GTO adds another 2.5 km/s, and reaching lunar orbit from LEO costs approximately 4 km/s. The Tsiolkovsky rocket equation links delta-v to propellant mass and specific impulse, making it the universal currency of spaceflight planning.