noun, adverb
Counts one occurrence of an action or event. Also used in sports for one round, game, bout, heat, or inning.
彼には一回だけ会ったことがある。
I've met him only once.
この試合は一回で決まる。
This match will be decided in one round.
一度 also means 'once' but is slightly more formal or literary. 一回 is more common in everyday counting.
二回 means 'twice' or 'two times', the direct counterpart for counting occurrences.
Compound of 一 (one) and 回 (counter for occurrences).