Specifically, the moment the
vehicle actually leaves the ground. On some
spacecraft, especially heavy ones, there can be a delay between
ignition and
liftoff. The
Space Shuttle, for example, first ignites the
main engines, and then a second or two later (after verifying that they're running) ignites the
Solid Rocket Boosters. This is because the main engines (being liquid-fuelled) can be shut off if something's wrong, but once the Solid Rocket Booster|SRB]'s are lit, you're
going, like it or not.
The term is also occasionally used to apply to aircraft, describing the moment the landing gear (or last part of the aircraft in contact with the ground) leaves the earth.