Be careful with this one. As far as I can ascertain, they are officially referred to as "the National Curriculum assessment" – so in official terms, SATs doesn't actually stand for anything. But according to Wikipedia, the reason they're referred to as SATs is because the government initially stated its intention to introduce Standard Assessment Tasks. Wikipedia then goes on to point out that "the term is variously believed to stand for Statutory Assessment Tests, Standard Attainment Tests, Standardised Achievement Tests and Standard Assessment Tests" – citing examples of each.
In my experience, if you get asked in a quiz what SATs stands for, the answer required is usually Standard Assessment Tasks. But because of the "various beliefs" referred to above, this question is always likely to be controversial!
© Haydn Thompson 2017