📘 Definition
Delta-v (ΔV) is the change in velocity a spacecraft must achieve to perform an orbital manoeuvre. It is the fundamental "currency" of spaceflight — every manoeuvre (launch, orbit raising, plane change, deorbit) has a delta-v cost, and a spacecraft's total available delta-v (determined by its propulsion system and fuel load via the Tsiolkovsky rocket equation) dictates what it can accomplish. For example, reaching LEO from Earth's surface requires approximately 9.4 km/s of delta-v. A Hohmann transfer from LEO to GEO requires about 3.9 km/s additional.