What is another term for a solemn promise?

A solemn promise is often most closely associated with the term "oath." An oath signifies a serious declaration or commitment, often made formally and with a sense of duty. It typically invokes a moral or legal obligation to uphold the stated promise and is frequently used in contexts that require integrity and trust, such as in legal settings or official ceremonies.

While a pledge and a commitment can both refer to a promise of varying degrees of seriousness, they do not carry the same weight or formal connotation as an oath. A guarantee, although it denotes assurance, often relates to the certainty of an outcome or fulfillment of terms rather than the moral or ethical obligation implied by an oath. Thus, the term "oath" stands out as the most fitting synonym for a solemn promise, emphasizing the gravity and formality associated with such a commitment.