Squadron Leader is a commissioned rank in the Royal Air Force

Royal Air Force

The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 and the air forces of many countries which have historical British influence. It is also sometimes used as the English translation of an equivalent rank in countries which have a non-English air force-specific rank structure. In these cases a squadron leader ranks above flight lieutenant

Flight Lieutenant

Flight lieutenant is a junior commissioned rank in the Royal Air Force and the air forces of many Commonwealth countries. It ranks above flying officer and immediately below squadron leader. The name of the rank is the complete phrase; it is never shortened to "lieutenant"...

 and immediately below wing commander

Wing Commander (rank)

Wing commander is a commissioned rank in the Royal Air Force and the air forces of many other Commonwealth countries...

.

Several abbreviations are used, including Sqn Ldr, Sqn.