The word "they" is a pronoun used in English that refers to a group of people or things that have already been mentioned or are understood from the context. It is commonly used as a subject in a sentence. Additionally, "they" is increasingly used as a singular pronoun to refer to a person whose gender is not specified or to represent non-binary individuals.
The word "they" has its roots in Old Norse "þeir," which was a plural pronoun used to refer to people. The usage has evolved over time, and it has been used in English since the 14th century.