Blast-off is the moment a spacecraft leaves the ground — the second the engines push hard enough that the rocket goes up. People also write blastoff as one word, and they use the two-word verb blast off for the action (“The rocket will blast off at noon”). It belongs to space talk and to the news: you hear the countdown, then blast-off. Children use it in play the same way. It feels exciting, not formal. Though actually, people who work on rockets more often say launch or lift-off; blast-off is the vivid, almost storybook word for that same instant of leaving the ground.