In English, the term "singular they" describes the practice of using of the pronoun "they" to refer to a single person rather than a plural group of people. It occurs most commonly when the gender of the individual being described is unknown. The use of singular they can be highly controversial.
Singular they occurs when the speaker does not know the number or gender of the individual or individuals being referred to. For instance, a speaker might say "if anyone wants to see me, they will have to wait." Normally, "they" would refer only to more than one individual, but in this case the speaker is using the pronoun to refer to an indeterminate number of people. One person or more than one may come, and the people may either be male or female.
In cases where the speaker refers to a single individual, but does not know whether the person is male or female, the speaker may use alternatives to singular they such as "he or she." However, in cases where both the number and gender are indeterminate, English lacks a suitable pronoun to describe the situation. Singular they can also describe objects which, although singular, have names which are grammatically plural. For instance, the phrase "I just bought new pants, and they tore the first time I wore them" uses "they" because "pants" is a plural noun, even though it describes only one object.
As can be seen in the previous examples, singular they is always plural, even when it refers to a singular individual. For instance, in the sentence "if a suspect is arrested, they have to be read their rights" uses the plural verb "have" even though the subject is singular. In the sentence "if a suspect is arrested, he or she has to be read his or her rights," the verb is different, even though the meaning of the sentence is exactly the same.
Singular they is a very controversial usage, although it has a long history. Authors such as Jane Austen used singular they, giving it a long pedigree, but many linguistic conservatives object to it because of the mismatch between the plural verb and its singular subject. Such commentators often prefer the use of "he" as a gender-neutral pronoun. Proponents argue that "he" as an indeterminate singular pronoun represents a bias against women. They also defend the indeterminate use of they as a natural development of the language.