Singular they is the use of ' (or its inflected forms, such as them or their) to refer to an entity that is not plural, or not necessarily plural. Though singular they is widespread in everyday English and has a long history of usage, debate continues about its acceptability. It typically occurs in these situations:
*Indeterminate gender – when they refers to an individual person of unknown or unspecified sex, as in, for example, "One student failed their exam". This usage is known as epicene they.
*Indeterminate number – when they has no definite antecedent, or can be interpre...