What does SAS stand for?
SAS stands for Special Air Service. It is the British Army's special forces, made up of one regular regiment, 22 SAS, and two reserve regiments, 21 SAS and 23 SAS, which together form part of United Kingdom Special Forces.
Where the name comes from
The name dates to the Second World War, when the unit was raised for raiding operations in North Africa. The 'Air Service' part was originally as much about deception as description — the intention was to suggest an airborne force larger than the one that actually existed. The name stuck long after the role changed.
Regular and reserve
22 SAS is the regular regiment, made up of full-time soldiers. 21 SAS and 23 SAS are reserve regiments: their members hold civilian jobs and train alongside them. All three sit under United Kingdom Special Forces. When people say 'the SAS' in conversation they usually mean 22 SAS; when someone talks about applying without leaving their job, they mean 21 or 23.
Why the distinction matters if you're thinking of applying
It changes everything about how you prepare. A reservist builds the fitness, the navigation and the hill hours around a working week, unsupervised, for months before anyone assesses them. That is the part most people underestimate, and it is the part I wrote about at length after applying to 21 SAS myself.
A note on accuracy
Entry criteria, standards and selection dates are set by the Army and change over time. Take anything current from the Army's own UK Special Forces Reserve pages; take everything below as one applicant's experience.
Get my 21 SAS reserve selection prep guide
What the stages are, what I was training to before I applied, and how I'd build the twelve weeks before a selection weekend. Written from my own application, free, no pitch.