Plighter

Part of speech: noun

Definitions

1. A person who makes a solemn pledge or promise, especially a betrothal or troth given to another
2. Someone who gives their word or binds themselves to a commitment, often in the sense of a formal engagement
3. One who solemnly vows or contracts a bond of faith, most often a marriage promise made to a beloved

Etymology: "Plighter" is about as transparent as English compounds get: it is "plight" with the agent suffix "-er," so a plighter is one who plights, that is, one who pledges. The more interesting part is the verb behind it. "Plight" as a verb means to pledge or promise solemnly, most famously in the phrase "to plight one's troth," meaning to pledge one's faithfulness, usually in marriage. That sense has faded from everyday use, which is why the noun now feels like a rare, almost archaic piece of vocabulary rather than an ordinary agent noun.