A person given as a pledge or security for the performance of the conditions of a treaty or similar agreement, such as to ensure the status of a vassal.
"The defeated lord sent his eldest son to the capital as a hostage to guarantee his continued loyalty to the king."
To give (someone or something) as a hostage to (someone or something else).
"The company decided not to release their new product, effectively giving it to critics as a hostage until they could fix the security flaws."