An adverb meaning that something happened by chance or coincidentally (similar to 'by chance' or 'as it happens').
偶然にも駅で昔の同級生に会った。
I happened to meet an old classmate at the station.
偶然にもその日は彼の誕生日だった。
Coincidentally, that day was his birthday.
偶然にも会議の日時が別の用事と重なってしまった。
By chance, the meeting time overlapped with another appointment.
偶然にも彼女も同じ本を持っていた。
As it happened, she also had the same book.