A person given or held to ensure that an agreement, demand, or treaty is kept or fulfilled.
Example:
The kidnappers released their hostage unharmed once all their demands were met.
Opponents in war sometimes exchange hostages to ensure that a truce or treaty remains unbroken. Hostages may also be taken by kidnappers or terrorists or rebels to use in bargaining for money or concessions. It may seem strange that the word hostage is connected with host and in fact with guest as well, since hostages are now unwilling guests, at the mercy of their hostile hosts.