An adverb meaning 'by any chance' or 'possibly', used to indicate a tentative possibility (similar to 'maybe' or 'perhaps').
ひょっとして、鍵を家に忘れたんじゃない？
By any chance, didn't you forget your keys at home?
ひょっとして彼はもう帰ったのかもしれない。
Perhaps he has already gone home.
ひょっとしてその噂は本当ですか？
Could that rumor possibly be true?