SAS, SBS, RMCs, SEAL teams, GSG-9, etc. could all be called "elite". It implies higher levels of skill and training than most units, and probably access to the best equipment. Some, such as the SAS, will be "invitation" units, ie you are asked to join if you're good enough, rather than applying to join yourself. I think elite status is relative, eg. the Republican Guard probably had far less training than your average British squaddie, but compared to the average Iraqi conscript they were far superior.
I'm not sure about Air Force units, though. I think some Battle of Britain squadrons could have been called elite by the end of 1940 - with so many pilots getting killed, the best would stand out because they would be aces (5 or more kills) and more importantly would still be alive.