Singular they is the use of they, or its inflected or derivative forms, such as them, their, or themselves, 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:
"Everyone returned to their seats."
"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."
A reason for its use is that English has no dedicated singular personal...