In spoken English, it is becoming common for people to use “they” while referring to the singular. A person might do this when they do not know the gender of the one they are speaking about. (Did you notice that I already did it?)
Instead of saying “he or she” and in order to avoid the cumbersome work of asking and researching about the gender of the person, one might just use “they”.
“I have no idea who the driver of this car is, but they should slow down or they’ll kill us all!”
When it comes to writing, however, it is not acceptable to use this singular “they”. It might cause confusion to the reader. Consider the following example:
“The author will acknowledge as many as they desire in their research article”.
It is not really clear whether these “they” and “their” refer to the author or to the publisher or to whom. This can be avoided if we use “he/she” as follows:
“The author will acknowledge as many as he/she desires in his/her research article”.
Using the singular “they” in writing is neither formal not academic. However, in informal writing, it can be accepted.