After the countdown, we cheered as the rocket reached liftoff.

liftoff 是复合名词，由动词短语 lift off（升空、起飞）合并而成。lift 意思是"举起、升起"，off 表示"离开、脱离"。liftoff 特指火箭或航天器从发射台升空的瞬间动作，即"发射升空"。