Traditionally it would be the year that pupils start their Standard Grade courses, or their Leaving Certificate in Ireland.
In Scotland, at the end of S3, pupils usually choose subjects in which they will work on in their Fourth year to sit National 3, 4 or 5.
Standard Grade courses are no longer taught in Scottish secondary schools.
The term can also refer to the third year of a university course.
This article relating to education in Scotland is a stub.