What is Hostage?

Legal Definition
A hostage is a person or entity which is held by one of two belligerent parties to the other or seized as security for the carrying out of an agreement, or as a preventive measure against war. However, in contemporary usage, it means someone who is seized by a criminal abductor in order to compel another party such as a relative, employer, law enforcement, or government to act, or refrain from acting, in a particular way, often under threat of serious physical harm to the hostage(s) after expiration of an ultimatum.

A person who seizes one or more hostages is known as a hostage-taker; if the hostages are present voluntarily, then the receiver is known as a host.
-- Wikipedia
Legal Definition
A person delivered into the possession of a public enemy in the time of war, as a security for the performance of a contract entered into between the belligerents.

2. Hostages are frequently given as a security for the payment of a ransom bill, and if they should die, their death would not discharge the contract. 3 Burr. 1734; 1 Kent, Com. 106; Dane's Ab. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
A person who is given into the possession of the enemy, in a public war, his freedom (or life) to stand as security for the performance of some contract or promise made by the belligerent power giving the hostage with the other.
-- Black's Law Dictionary
Legal Definition
An inn; one held by an enemy as security for the performance of demands.
-- Ballentine's Law Dictionary