vow

To vow is to make an earnest promise or pledge. People getting married often vow to stick together “till death do us part,” while a presidential candidate might vow to lower taxes.

You can use vow as both a noun and a verb: A vow is a solemn declaration; to vow is the act of making that promise. Vows are meant to be serious, sacred things, even vows made with the best intentions can be broken. Wrote American short-story writer and poet Dorothy Parker, “By the time you swear you are his, shivering and sighing, And he vows his passion is infinite and undying—Lady, make a note of this: One of you is lying.”