They

Singular

The singular they is the use of this pronoun as a gender-neutral singular rather than as a plural pronoun. The Oxford Dictionaries have an article on the usage, saying that it dates back to the 16th century.^[1]

The singular pronoun they can be found in formal or official texts. For example, a 2008 amendment to the Canadian Criminal Code contains the following text:

if a peace officer has reasonable grounds to believe that, because of their physical condition, a person may be incapable of providing a breath sample... (subparagraph 254(3)(a)(ii))

In an article published in The New York Times Magazine in 2009, Patricia T. O'Conner and Stewart Kellerman wrote:

Anne Fisher (1719-78) [an 18th-century British schoolmistress and the first woman to write an English grammar book] was not only a woman of letters but also a prosperous entrepreneur. She ran a school for young ladies and operated a printing business and a newspaper in Newcastle with her husband, Thomas Slack. In short, she was the last person you would expect to suggest that he should apply to both sexes. But apparently she couldn’t get her mind around the idea of using they as a singular.

...

Meanwhile, many great writers — Byron, Austen, Thackeray, Eliot, Dickens, Trollope and more — continued to use they and company as singulars, never mind the grammarians. In fact, so many people now use they in the old singular way that dictionaries and usage guides are taking a critical look at the prohibition against it. R. W. Burchfield, editor of The New Fowler’s Modern English Usage, has written that it’s only a matter of time before this practice becomes standard English: “The process now seems irreversible.” Merriam-Webster’s Collegiate Dictionary (11th ed.) already finds the singular they acceptable “even in literary and formal contexts,” but the Usage Panel of The American Heritage Dictionary of the English Language (4th ed.) isn’t there yet. ^[2]

Generic

The pronoun they can also be used to refer to unspecified people in some often vaguely defined group, as in In Japan they drive on the left. It often refers to the authorities, or to some perceived powerful group, sometimes sinister: They don't want the public to know the whole truth.

It can also be used as a gender neutral third person singular pronoun. This idiomatic use avoids formalising the vagueness or unknown fact by not using the formal phrase, "he or she." For example, formally "he or she drove over the body and disappeared," informally "they (singular) drove over the body and disappeared."