they 
(thā)
(thā)pron.
1. Used to refer to the ones previously mentioned or implied.
2.
a. Used to refer to the one previously mentioned or implied, especially as a substitute for generic he: Every person has rights under the law, but they don't always know them.
b. Used as a singular personal pronoun for someone who does not identify as either male or female. See Usage Note below.
3.
a. Used to refer to people in general.
b. Used to refer to people in general as seen in a position of authority.
[Middle English, from Old Norse their, masculine pl. demonstrative and personal pron.; see to- in the Appendix of Indo-European roots.]
Usage Note: The use of the plural pronouns they, them, themselves, or their with a grammatically singular antecedent dates back at least to 1300, and such constructions have been used by many admired writers, including William Makepeace Thackeray ("A person can't help their birth"), George Bernard Shaw ("To do a person in means to kill them"), and Anne Morrow Lindbergh ("When you love someone you do not love them all the time"). Despite the apparent grammatical disagreement between a singular antecedent like someone and the plural pronoun them, the construction is so widespread both in print and in speech that it often passes unnoticed. There are several reasons for its appeal. Forms of they are useful as gender-neutral substitutes for generic he and for coordinate forms like his/her or his or her (which can sound clumsy when repeated). Nevertheless, the clash in number can be jarring to writers and readers, and many people dislike they with a singular antecedent. This includes much of the Usage Panel, though their resistance has declined over time. Resistance remains strongest when the sentence refers to a specific individual whose gender is unknown, rather than to a generic individual representative of anyone: in our 2015 survey, 58 percent of the Panel found We thank the anonymous reviewer for their helpful comments unacceptable. A sentence with a generic antecedent, A person at that level should not have to keep track of the hours they put in, was rejected by 48 percent (a substantial change from our 1996 survey, in which 80 percent rejected this same sentence). As for the use of they with antecedents such as anyone and everyone, pronouns that are grammatically singular but carry a plural meaning, by 2008, a majority of the Panel accepted such sentences as If anyone calls, tell them I can't come to the phone (56 percent) and Everyone returned to their seats (59 percent). For those who wish to avoid the apparent clash of number, some of these sentences can be recast in the plural: People at that level should not have to keep track of the hours they put in. Unfortunately, the option is unavailable when the referent must be singular: Lindbergh's sentence cannot be recast as When you love people, you do not love them all the time without drastically changing its meaning, nor can the sentence about the anonymous reviewer. · The recent use of singular they for a known person who identifies as neither male nor female remains controversial; as of 2015 only 27 percent of the Panelists accepted Scout was born male, but now they do not identify as either traditional gender. With regard to this last sentence, the Panel's responses showed a clear generational shift: the approval rate was 4 percent among Panelists born before 1945 and 40 percent among Panelists born later. See Usage Notes at anyone, he1.
Word History: Incredible as it may seem, the English pronoun they is not a native English pronoun. They comes from Old Norse and is a classic example of the profound impact of that language on English: because pronouns are among the most basic elements of a language, it is rare for them to be replaced by borrowings from foreign sources. The Old Norse pronouns their, theira, theim worked their way south from the Danelaw, the region governed by the Old Norse-speaking invaders of England, and first appeared in English about 1200, gradually replacing the Old English words hīe, hīora, him. The nominative or subject case (modern English they) seems to have spread first. William Caxton, who brought the printing press to England, uses they, hir, hem in his earlier printed works (after 1475) and thei, their, theim in his later ones. This is clear evidence of the spread of these Norse forms southward, since Caxton did not speak northern English natively (he was born in Westminster). The native English plural him or hem may well survive, at least colloquially, in modern English 'em, as in "Give 'em back!"