A person to whom one is engaged to be married; a gender-neutral term for someone who is engaged.
彼は婚約者を家族に紹介した。
He introduced his fiancée to his family.
婚約者と結婚式の日程を決めた。
I set the date of the wedding with my fiancé/fiancée.
彼女の婚約者は来月海外へ転勤するらしい。
Apparently her fiancé is being transferred abroad next month.