How to say "onset" in Japanese

• 最初さいしょJLPT N4approx.Common
  no-adjective, adverbCovers the 'beginning; outset' sense of onset: the start of a period, event, or process. The common, default choice; say 最初に or 最初の段階で when you mean 'at the onset'.
  その考かんがえは最初さいしょのうちはばかげているように思おもえた。↗

  The idea seemed absurd at first.
• 発来はつらい
  nounGlossed as 'onset; outbreak', but a less common word. It suits formal contexts describing the arrival or outbreak of something, and is often not the word you want for an ordinary 'beginning'.

Use 最初 for the everyday sense of the very beginning or outset of something; 発来 is a less common word covering onset or outbreak, suited to formal descriptions of something arriving or emerging.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 最初さいしょbeginning
• 発来はつらいonset