| Definition -
The use of the pronoun they and its inflected forms as a singular, e.g. Everyone has to remove their shoes.
1. Usually, people say this to avoid
(1) sounding sexist
(Everyone has to remove his shoes) or
(2) sounding awkward
(Everyone has to remove her or his shoes) or
(3) sounding sexistly awkward
(Everyone has to remove his or her shoes).
2. "The singular they had emerged by the mid-14th century and is common in everyday spoken English, but its use has been the target of criticism since the late nineteenth century."
Source: Wikipedia s.v. singular_they
1. I have difficulty with this one: I like agreement in number. I understand the motivation for this sloppiness, but think it would be even more clever to have a new word for a singular 'they'.