A term describing the beginning of the first stage of the propulsion process of a rocket or launcher. T–0 is often used in countdowns to indicate the moment of lift-off. Some rockets take off at engine ignition, as in the case of hypergolic propellants which ignite spontaneously on contact, while some lift off a few seconds after engine ignition, allowing the engine to build up to full thrust; in each case this is termed T–0. However, T–0 is also sometimes and confusingly used to mark the point of engine ignition, and actual lift-off may therefore occur at T + a number of seconds.