noun
A solemn promise, often used in formal, religious, or personal commitment contexts.
彼は神に誓いを立てた。
He swore an oath to God.
結婚の誓いを交わす。
Exchange wedding vows.
約束 is a general promise or appointment, while 誓い is a more solemn, often lifelong or sacred vow.
Derived from the verb 誓う (ちかう, to swear), with the nominalizing suffix い.