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'They' is a third person singular pronoun. It has been in use to denote an singular individual of unknown gender for hundreds of years (since ~14th century).

"With an antecedent referring to an individual generically or indefinitely (e.g. someone, a person, the student), used esp. so as to make a general reference to such an individual without specifying gender"[0]

"Often used in reference to a singular noun made universal by every, any, no, etc., or applicable to one of either sex (= ‘he or she’)."[1]

[0] - https://www.oed.com/view/Entry/200700 - current OED entry

[1] - https://www.oed.com/oed2/00251026 - 2nd edition OED entry from 1989