Rocket launch glossary

The words of a launch in one line each: what you hear in a countdown and on a webcast, how boosters come back, and where satellites go. Launch pages and the globe show these lines too when you point at a term or tap it.

Before liftoff

NET: No earlier than: the earliest a launch can happen. Dates often move later, rarely earlier.
Go, TBC, TBD: Go: the launch provider has confirmed the date and time. TBC: a date is announced but not yet firm. TBD: no firm date yet.
Hold: The countdown is paused, for the weather, a technical check or a boat in the danger area; it resumes, or the attempt is scrubbed.
Scrub: A launch attempt called off before liftoff; the rocket tries again on a later day or later in the window.
Launch window: The span of time in which the rocket can lift off and still reach its orbit. Some are a single second (instantaneous), others last hours.
T−, T+: T− counts down the time left before liftoff (T−0); after liftoff the clock counts up as T+.
Propellant load: Filling the rocket with fuel and oxidizer, usually liquid oxygen (LOX), in the last hour or so before liftoff.
Engine chill: Cold propellant flows through the engines to cool them, so it doesn't boil when it rushes in at ignition.
Tank pressurization: The tanks are brought up to flight pressure, which keeps the propellant flowing to the engines and stiffens the rocket.
Static fire: A test firing of the engines with the rocket held down on the pad, days before launch.

On the way up

Max-Q: Maximum dynamic pressure: about a minute after liftoff, the moment the air pushes hardest on the rocket as speed rises and the air thins.
BECO: Booster engine cut-off: the boosters' engines (or Super Heavy's) shut down just before they separate.
MECO: Main engine cut-off: the first stage's engines shut down, seconds before the stages separate.
Hot staging: The upper stage lights its engines while still attached to the booster and pushes away from it; Starship stages this way.
Kick stage: A small extra stage that gives the satellite its last push into the exact orbit it needs.
Stage separation: The spent stage (or the strap-on boosters) lets go of the rest of the rocket, which flies on lighter.
SES: Second engine start: the upper stage's engine lights after separation. SES-2 is a later relight, to change orbit or come back down.
Fairing: The nose cone that shields the satellite through the air. Once above most of the atmosphere it splits in two and falls away.
SECO: Second engine cut-off: the upper stage's engine shuts down, usually as it reaches orbit (SECO-1); SECO-2 ends a later burn.
Orbit insertion: The moment the spacecraft flies fast enough, about 28,000 km/h in a low orbit, to keep falling around the Earth instead of back to it.
Payload deployment: The satellites or the spacecraft are released from the upper stage to go on alone.
TLI: Trans-lunar injection: the burn that takes a spacecraft out of Earth orbit and sends it to the Moon.
Deorbit burn: An engine burn against the direction of flight that slows the spacecraft just enough to fall back into the atmosphere.

Coming back

Flip: The stage turns around in flight: a booster swings engines-first for its burns; Starship's Ship swings from belly-first to upright just before landing.
Boostback burn: Right after separation the booster fires against its course to head back toward the launch site or a closer landing spot.
Entry burn: A burn as the falling booster meets the thicker air, slowing it so the heat and pressure don't damage it.
Reentry: Coming back into the atmosphere at up to about 27,000 km/h: the air ahead is squeezed and glows, heating the heat shield to well over 1,000 °C.
Peak heating: The moment of reentry when the heat on the vehicle is greatest.
Supersonic, subsonic: Faster or slower than the speed of sound, about 1,200 km/h near the ground. Coming down, a stage slows below it shortly before landing.
Landing burn: The last burn, which slows the booster or the ship from hundreds of km/h to a gentle touchdown or a catch.
Tower catch: Instead of landing on legs, a Starship stage is caught by two arms on the launch tower (the chopsticks), so it can fly again sooner.
Drone ship: A ship without a crew at sea where a booster lands when it lacks the fuel to fly back to land; SpaceX's are called ASDS.
RTLS: Return to launch site: the booster flies back and lands near where it launched, on a landing zone.
Splashdown: Coming down in the sea: crew capsules under parachutes, or a test stage set down softly on the water as planned.
Landing: A stage or spacecraft touching down so it can be recovered, and for boosters flown again.
Reusable booster: A first stage that lands and flies again; the most flown Falcon 9 boosters have flown over 30 times.

Orbits

Low Earth orbit (LEO): Up to about 2,000 km high, one lap every 90 minutes or so. The ISS, Hubble and Starlink fly here.
Sun-synchronous orbit (SSO): A low orbit over the poles that passes each place at the same local time every day, so Earth-watching satellites see it in the same light.
Polar orbit: An orbit that passes over or near both poles, so the satellite sees the whole Earth turn beneath it.
Medium Earth orbit (MEO): Between low orbits and 35,786 km; the navigation satellites (GPS, Galileo, GLONASS, BeiDou) fly at about 20,000 km.
Geostationary transfer orbit (GTO): A long oval from a low orbit up to geostationary height; the satellite later fires its own engine there to make the orbit round.
Geostationary orbit (GEO): 35,786 km above the equator, where a satellite circles once a day and seems to hang over one spot; TV and weather satellites live here.
Elliptical orbit: An oval orbit much higher at one end than the other; some linger for hours over the far north this way.
Lunar orbit: An orbit around the Moon, or a path to it, reached after a trans-lunar injection burn.
Lagrange point: A spot where the pull of the Sun and the Earth lets a spacecraft keep pace with the Earth; telescopes like JWST sit at L2, 1.5 million km away.
Heliocentric orbit: An orbit around the Sun rather than the Earth, for probes on their way to other planets or asteroids.
Suborbital: A flight that reaches space but not the speed to stay there, so it comes back down; New Shepard flies this way.
Inclination: The angle between an orbit and the equator. A rocket can't reach an inclination lower than its launch site's latitude without spending extra fuel.
Apogee, perigee: The highest and the lowest points of an orbit around the Earth.

See them in action: the 3D globe replays each flight moment by moment, and every launch page has its timeline.