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再 is the native Sino-Japanese prefix meaning 're-' used with kanji compounds like 再開, while リ is a loanword prefix used with katakana words like リセット.
再 (さい, 're-/again') and 最 (さい, 'most') share the same reading but have completely unrelated meanings, so learners can mix up which kanji to write.
際 (occasion, time) and 再 (again, re-) are pronounced identically as さい but have completely unrelated meanings, so learners must rely on context or kanji to tell them apart.
revival; restoration to life; regeneration · rehabilitation; reformation; turnaround · recycling; reclamation; recovery · playback; playing audio or video; views · biological regeneration; regrowth · rebirth; reincarnation · memory recall; retrieval
reappearance; return; revival · reproduction; reenactment; recreation
reopening; resumption; restarting
again; once more; a second time
meeting again; reunion
Sino-Japanese prefix from Middle Chinese 再 (tsojH), meaning 'twice' or 'again'.
reorganization; reshuffle; restructuring
resale; reselling
return; coming back; revisit · second coming; second advent; reincarnation
recurrence; revival; resuscitation
re-election
repeat performance; revival; encore presentation · reprising a role; playing the same role again · recapitulation
comeback; recovery; rally; restoration
reconsideration; rethinking; review
repeatedly; again and again
revival; restoration; resuscitation
often; frequently; again and again
restart; resume
return; recurrence · recursion · reflexivity
retrial; reopening of a case · review; reexamination
Second Coming; Second Advent
second offense; reoffending; recidivism
repost; reprint; redisplay
rematch; second match; return match
revisit; visiting again
reappointment; reappointing
reprint; second edition
resend; retransmit