How to say "spontaneous" in Japanese

• 自発的じはつてきCommon
  na-adjectiveThe core word for 'spontaneous' in the sense of voluntary, self-initiated action: she offered help without being asked. Emphasizes the person's own will rather than outside pressure.
  彼かれは自発的じはつてきに降伏こうふくした。↗

  He surrendered of his own accord.
• 任意にんいCommon
  no-adjective, na-adjectiveCovers 'spontaneous' only loosely, in the sense of freely chosen or arbitrary. It really means 'optional' or 'at one's discretion', so it is seldom the word you want for spontaneous behavior.
• 自然しぜんJLPT N3approx.Common
  na-adjective, nounSuits 'spontaneous' when something happens naturally or without deliberate effort, such as an unforced reaction or an automatic response. The emphasis is on naturalness rather than willingness.
• 無意識むいしきCommon
  na-adjective, no-adjectiveFits 'spontaneous' meaning unconscious or involuntary, done without awareness, like a mechanical or automatic gesture. It does not convey willing self-initiation, so it is wrong for deliberate spontaneous acts.
  お母かあさんは無意識むいしきに椅子いすをつかんだ。↗

  Unconsciously my mom took hold of the chair.

Use 自発的 for willing, self-initiated actions, 自然 for unforced or automatic naturalness, 任意 for something optional or chosen freely, and 無意識 for unconscious, involuntary behavior.

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

• 自発的じはつてきspontaneous
• 任意にんいoptional
• 内発的ないはつてきspontaneous
• 特発性とくはつせいidiopathic
• 無意志むいしnon-volitional
• 自然しぜんnatural
• 無意識むいしきunconscious