Topic summary
Singular they

Singular they is an epicene (gender-neutral) third-person pronoun in English. It typically occurs with an indeterminate antecedent, to refer to an unknown person, or to refer to every person of some group, in sentences such as:
This use of singular they had emerged by the 14th century, about a century after the plural they. Singular they has been criticized since the mid-18th century by prescriptive commentators who consider it an error. Its continued use in modern standard English has become more common and formally accepted with the move toward gender-neutral language. Some early-21st-century style guides described it as colloquial and less appropriate in formal writing. However, by 2020, most style guides and English-learning websites, such as the APA, AP, CMOS, British Council etc. accepted the singular they as a personal pronoun.
In the early 21st century, use of singular they with known individuals emerged for non-binary people, as in, for example, "This is my friend, Jay. I met them at work." They in this context was named Word of the Year for 2015 by the American Dialect Society, and for 2019 by Merriam-Webster. In 2020, the American Dialect Society also selected it as Word of the Decade for the 2010s.