The moment when a spacecraft or rocket leaves the ground and begins its journey into space or the air.
From 'lift' (to raise up) plus 'off' (away), a compound that emerged in the early 1900s with aviation. Became standard terminology with the Space Age in the 1960s.
Liftoff is one of the few technical terms that managed to enter common speech—people who've never been to space understand it perfectly because the compound makes logical sense, showing how compound words are some of the most successful new words in English.
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