In the UK England (many thanks gkAndy), short for Statutory Assessment Tests. A series of tests in English, Maths and Science taken at the end of each Key Stage (Department of Education and Skills jargon for the different levels of learning in schools). They are therefore taken in Year 2 (English only), Year 6 and Year 9.
The two earlier English tests are taken in the form of multiple-choice comprehension questions, whereas the Year 9 tests are more essay-based (similar to the GCSE tests taken in Year 11).
Although much is made of the tests from the pupils' perspective, they are utterly meaningless in any official context, and are used only to measure the progress of students - and of course the performance of their teachers as well. How anyone can still believe exams like this are an accurate way of measuring intelligence or aptitude for anything escapes me.