Real Dictionary

guard
A guard is a specially organized group of people, such as soldiers or police officers, who protect or watch someone or something.
countable noun: A guard is someone such as a soldier, police officer, or prison officer who is guarding a particular place or person.
verb: If you guard a place, person, or object, you stand near them in order to watch and protect them.
verb: If you guard someone, you watch them and keep them in a particular place to stop them from escaping.

guard in American English
the act or duty of guarding; careful watch; wariness; defense; protection
to keep safe from harm; watch over and protect; defend; shield
to keep watch; take precautions (against)
intransitive verb: to take precautions
noun: a person or group of persons that guards, protects, or keeps a protective or restraining watch
transitive verb: to keep safe from harm or danger; protect; watch over

guard in British English
noun: a person or group that keeps a protecting, supervising, or restraining watch or control over people, such as prisoners, things, etc
verb: to watch over or shield (a person or thing) from danger or harm; protect