A prisoner who is held by one party to insure that another party will meet specified terms WordNet 3.0 & Open English WordNet

A hostage is a person seized by an abductor in order to compel another party, one which places a high value on the liberty, well-being and safety of the person seized—such as a relative, employer, law enforcement, or government—to act, or refrain from acting, in a certain way, often under threat of serious physical harm or death to the hostage after expiration of an ultimatum. Wikipedia

Person or entity held by a captor. Wikipedia Disambiguation

Act of taking and holding a hostage or hostages Wikidata

Person/entity held by a belligerent party to another or seized for carrying out agreement Wikidata

A person or entity which is held by a captor in order to compel another party to act or refrain from acting in a particular way. OmegaWiki

A person seized in order to compel another party to act (or refrain from acting) in a certain way, because of the threat of harm to the hostage. Wiktionary