Squadron leader (Sqn Ldr in the RAF ; SQNLDR in the RAAF and RNZAF; formerly sometimes S/L in all services) is a commissioned rank in the Royal Air Force 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.
Squadron leader has also been used as a cavalry command appointment (UK) and rank (France) since at least the nineteenth century. In Argentina it is used as a command appointment by both the army's cavalry and by the air force's flying units. The cavalry rank of squadron leader in France is also an OF-4 equivalent to a major, and the cavalry appointment of squadron leader in the UK generally corresponds to this rank as well.
The rank originated in the British Royal Air Force and was adopted by several other air forces which use, or used, the RAF rank system.
On 1 April 1918, the newly created RAF adopted its officer rank titles from the British Army, with Royal Naval Air Service lieutenant commanders and Royal Flying Corps majors becoming majors in the RAF. In response to the proposal that the RAF should use its own rank titles, it was suggested that the RAF might use the Royal Navy's officer ranks, with the word "air" inserted before the naval rank title. For example, the rank that later became squadron leader would have been air lieutenant commander. However, the Admiralty objected to this modification of their rank titles. The rank title squadron leader was chosen as squadrons were typically led by RAF majors and the term squadron commander had been used in the Royal Naval Air Service. The rank of squadron leader has been used continuously since 1 August 1919.