Merriam-Webster added a new definition of the pronoun "they" to its Webster's Dictionary. It says "they" also refers to a "single person whose gender identity is non-binary". These are people who identify neither as male nor female. Many places use a gender-neutral option on official forms. Many local governments, schools and airlines have the choice of "X" for people who do not consider themselves as male or female, and for gender fluid people.
Merriam-Webster said using "they" for "he" or "she" recognizes people who did not "conform to an expected gender expression, or who seemed to be neither male nor female". It says the word "they" was used as a singular pronoun in the 1300s. Shakespeare used it in this way in the 17th century. Also, the 1898 play Antony and Cleopatra has the words: "No man goes to battle to be killed....But they do get killed." Many people use the singular "they" today.