They (pronoun)

1. Used to refer to two or more people or things previously mentioned or assumed.
2. Used as the subject of a verb to refer to two or more antecedents.
3. Used as the object of a verb or preposition to refer to two or more antecedents.
4. Used to refer to a person of unspecified gender.
5. Used as an indefinite pronoun to refer to a person.

Origin:

Middle english, from old norse þeir, masculine and neuter nominative plural of sá, pronoun and demonstrative.

Examples:

1. They are leaving tomorrow.
2. I saw them at the store.
3. They're my friends.
4. The student is going to meet with their advisor.
5. They say it's going to rain.