ASH – Air to Surface H or AI Mk XV (U.S AN/APS4). Centimetric airborne air-air radar derived from ASV operating at 3 cm wavelength at a frequency of 10 GHz. Used by 100 Group Mosquitos; postwar in the Sea Hornet N.F. Mark 21.
ASV – Air to Surface Vessel radar. A 1.5 metre radar that could detect surfaced submarines at up to 36 miles.
G-H – British radio navigation system used for blind bombing - from TRE
GEE – British radio navigation system forerunner of LORAN - from TRE
Granite - a system of coloured flares and rockets deployed by the Royal Observer Corps to warn aircraft of high ground in fog and mist. In certain parts of the country the flares were used to steer fogbound aircraft to FIDO equipped airfields.
Grocer (later "Ground Grocer") – ground-based version of Airborne Grocer
H.F. D/F (High Frequency Direction Finding)  – provided a radio position fix for the RAF up to 100 miles from the transmitters in Britain. The system was based on voice communications, and was used for aircraft to find their home bases. With the development of GEE, its primary function ceased, but it remained in use until the end of the war as a backup system and a communications system between aircraft and their bases.
H2S – British ground mapping radar to see target at night and through cloud cover - from TRE
H2X – American 10 GHz ground mapping radar, higher frequency development of British H2S
Jay beams – were introduced partly as a deception to help to confuse the Germans over the use of GEE. It was nevertheless just as useful as a homing beacon. A number of transmitters, from Lossiemouth to Manston in Kent transmitted on slightly different frequencies transmitted a narrow beam across the North Sea using a S.B.A. (Standard Beam Approach) transmitter, receivers for-which were fitted to all British bombers and could be received over a range of 350 miles at 10,000 feet. Once a bomber found a beam it could fly down it back to Britain. In late 1943, all but two beams were closed down with these final two shutting down towards the end of 1944 because GEE could do the job better and their use to deceive the Germans was by now redundant.
Naxos – FuG 350, German H2S detection and homing device, not capable of detecting the Americans' similar, higher frequency (10 GHz) H2X radar
Neptun – FuG 216, -217 & -218, German high-VHF-band (125 to 187 MHz) night fighter AI radar, introduced mid/late 1944, generally used the Hirschgeweih antenna setup of Lichtenstein VHF-band radar sets with shorter dipole elements, as a replacement for the compromised Lichtenstein SN-2 90 MHz AI equipment
Gardening – RAF operations dropping mines in strategic sea lanes, usually at the request of the CoS Naval Liaison Officer based at High Wycombe. As a spinoff, Bletchley Park cryptanalysts used German reports of Gardening activities as cribs in the decryption of Enigma messages.
Kammhuber Line – British name for the German Himmelbett radar controlled air defence system
Operational research – the statistical analysis of results and anomalies, some caused by the use of previously unknown German electronic equipment or tactics based on the equipment
Wilde Sau (Wild Boar) – German freelance night fighters, i.e. not parked round a visual beacon like the Zahme Sau (Tame Boar) fighters
Zahme Sau (Tame Boar) – German tactic of guiding a night fighter 'parked' round a visual beacon, onto the incoming bomber stream by radar-assisted ground commentary