What is described as a solemn promise?

The term that describes a solemn promise is "Oath." An oath is a formal declaration or a promise, often invoking a divine witness, to speak the truth or fulfill a duty. It carries a weight of seriousness and commitment, typically associated with important responsibilities, such as in legal contexts or solemn occasions like weddings or public offices. By taking an oath, an individual signifies their earnest intention to adhere to the promise made, reflecting a profound moral obligation. This distinguishes it from other terms like agreement, commitment, or assurance, which may not necessarily have the same level of formality or gravity.