An adverb meaning unintentionally or involuntarily; doing something instinctively or without thinking.
その映画を見て思わず泣いてしまった。
I watched that movie and involuntarily started to cry.
驚いて思わず声を上げた。
I was startled and unintentionally let out a sound.
母の言葉に思わず笑ってしまった。
I couldn't help but laugh at my mother's words.