Wiktionary
n. A rank in the RAF, between that of wing commander and air commodore, equivalent to an army colonel
WordNet
n. a commissioned officer (especially one in the Royal Air Force) equivalent in rank to a colonel in the army
Wikipedia
Group captain (Gp Capt or Grp Cpt in the RAF, IAF and PAF, GPCAPT in the RNZAF and RAAF; formerly sometimes G/C in all services) is a senior commissioned rank which originated in and continues to be used by the Royal Air Force. The rank is also used by the air forces of many countries which have historical British influence and it is sometimes used as the English translation of an equivalent rank in countries which have a non-English air force-specific rank structure. It ranks above wing commander and immediately below air commodore. The name of the rank is the complete phrase, and is never shortened to "captain".
It has a NATO ranking code of OF-5, and is equivalent to a captain in the Royal Navy or a colonel in the British Army or the Royal Marines. Group captain is the rank usually held by the station commander of a large RAF station.
The equivalent rank in the Women's Auxiliary Air Force, Women's Auxiliary Australian Air Force, Women's Royal Air Force (until 1968) and Princess Mary's Royal Air Force Nursing Service (until 1980) was "group officer". The equivalent rank in the Royal Observer Corps (until 1995) was "observer captain", which had a similar rank insignia.
Usage examples of "group captain".
There were four of us, a major, a group captain, a rubber planter and me.
Chief of Staff Sindlinger and Group Captain Cady were pleased with the way American/British co-operation was shaping up.
The Group Captain took the hint and got up, closing the file of papers they'd been dealing with.
Marshall went back to the Headquarters office, made his apologies to the Group Captain, and asked for air transport.