再三 (zài sān) means 'again and again, repeatedly', often used for actions done several times. It differs from 一再 (yī zài) in that 再三 can appear after the verb as a complement (e.g., 考虑再三 'after repeated consideration'), while 一再 is strictly pre-verbal (e.g., 一再强调 'emphasized repeatedly'). Both are common in formal writing and speech.