"Singular" they is a popular, non-technical expression for uses of the pronoun they (an its inflected forms) when plurality is not required by the context. The Chicago Manual of Style notes: "On the one hand, it is unacceptable to a great many reasonable readers to use the generic masculine pronoun ('he' in reference to no one in particular). On the other hand, it is unacceptable to a great many readers either to resort to nontraditional gimmicks to avoid the generic masculine (by using 'he/she' or 's/he.' for example) or to use 'they' as a kind of singular pronoun."