Singular they is the use in English of the pronoun they, or its inflected or derivative forms, such as them, their, themself, or themselves, as a gender-neutral pronoun to refer to a single person or an antecedent that is grammatically singular. It typically occurs with an antecedent of indeterminate gender, as in sentences such as:
"Somebody left their umbrella in the office. Would they please collect it?"
"The patient should be told at the outset how much they will be required to pay."
"But a journalist should not be forced to reveal their sources."
The singular they had emerged by the 14th century...