[singular] Used to refer to a person of unspecified sex (ed: or gender): 'ask a friend if they could help'
People in general: 'the rest, as they say, is history'
Gender-neutral singular pronoun for a known person, particularly as a nonbinary identifier.
(Also note that singular 'they' has been voted Word of the Year by the American Dialect Society)
MYTH Singular 'they' is acceptable by both modern and historical English standards. It has used by notable authors such as Shakespeare, C. S. Lewis, Chaucer, Jane Austen, Oscar Wilde and numerous others. The 3rd edition of the New Fowler's guide permits the usage of singular 'they', as do various other notable English grammar guides.
MYTH Take a moment to think about how you would describe someone whose gender you didn't know. Ask yourself how you would form a sentence in which that person goes swimming. "They went to the pool, and then they went swimming." It's surprisingly natural when you're not thinking about it, so why would it be any less so when you are?
MYTH Is it rude to refer to someone who is clearly a guy as 'she', or vice versa? Of course it is! But not all situations are so clear-cut. If you don't know someone's gender, trying to make an assumption about it so you can use a common gendered pronoun ('he', 'she') is far ruder than using a neutral alternative.
As previously discussed, using 'they' as a pronoun in these cases is so natural that people are unlikely to even notice when you've done so; it's unlikely to make them uncomfortable. If their preferred pronoun is 'they', though, it will definitely be noticed, but certainly in a positive light!
While this page discusses the use of singular 'they' as a gender neutral pronoun, there are many others. Which pronoun a person prefers is a personal choice, and different people will have different preferences as to which gender neutral pronoun they use and why. Alternative gender-neutral pronouns are just as valid as any others. If you want any more information on other gender neutral pronouns, have a look at Wikipedia - although not even their list is exhaustive!
'They' is our favourite gender-neutral pronoun. This doesn't make it better than the others, but here is why we like it.