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This glossary of historic aviation terms is reproduced from 'The Spotters Glossary' which appeared in 'The Aeroplane Spotter' from January 1940 onwards. 'The Aeroplane Spotter' was the first journal devoted exclusively to the study and practice of aircraft recognition.

We are pleased to have been able to reproduce the entire glossary from A-Z, offering a unique insight into aviation terminology (and therefore aviation history and technology) prior to and during the 1940s.

NB: In the interests of clarity we have substituted current equivalents where words and phrases used in the original publication have fallen from general use (eg, 'aero motor' has been replaced by 'aero engine').

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Observation Mirror: An engraved mirror used for bombing training in the same way as a camera obscura. The bomber flies over the mirror, which is suitably mounted on the ground. A light is flashed in the aeroplane at the release point of the bomb and this is plotted on the mirror. In this way the accuracy of the "bombing" can be determined.

Obstruction Angle: The angle between the horizontal and a line joining the highest obstruction in the approaches to an aerodrome to the nearest point of the appropriate runway.

Obstruction Light: A light indicating the presence of an object which is dangerous to an aircraft in flight.

Obturator Ring: An L-section ring fitted round a piston to maintain a gas-tight seal between the piston and the cylinder wall.

Occlusion: A meteorological term for the "front" (that is, junction between masses of warm and cold air) which remains after a cold front has met a warm front and the latter has been forced upwards.

Occulting Light: A flashing light in which the light period is distinctly longer than the dark period.

Octane Number: The quality of a fuel to produce power in a high-performance aero-engine without pre-ignition through high compression. The higher the octane number the greater the power that can be taken from the fuel, provided that the engine is designed to take advantage of it. The present highest octane number in use is 100. In peacetime ordinary motor fuel was about 67 octane. (NB: Contemporary reference circa 1940)

Omni-directional Beacon: A ground radio transmitter which is non-directional and is used for navigational purposes by aeroplanes with direction finders. The aeroplane's radio operator recognises the beacon from its code signal and then finds its bearing from the aeroplane with his directional receiver.

Open-jet Wind Tunnel: A wind tunnel in which the air stream passing the body under test is not enclosed, but is in the form of a free jet rather like water squirting from a hose.

Opposed-piston Engine: An engine in which the compression is effected by two pistons in the same cylinder acting in opposition. The Junkers Jumo 205 C.1. aero-engine works on this principle.

Optimum Angle: (1) The gliding angle at which the least height is lost in proportion to the ground covered. (2) The angle of attack of an aerofoil at which the ratio of lift to drag is greatest.

Ornithopter: A mechanically driven aerodyne whose lift in flight is obtained by the action of flapping wings.

Orographic Rain: Rain caused by rising ground in the path of moisture-laden wind.

Oswald Watt Gold Plaque: A gold medal presented by the late Colonel W. Oswald Watt for yearly award to the airman who performed the most outstanding performance during the year in the Commonwealth of Australia, or to the Australian-born airman who performs the most brilliant feat outside Australia. The plaque was first awarded in 1921 to F.S. Briggs.

Outer Area: A circular zone round an airport within which the obstructions must not be above a certain height.

Outer Cover: The external covering of the hull of a rigid airship.

Outer Marker-beacon: A radio beacon comprising part of a blind approach installation which defines a predetermined distance from the aerodrome boundary. As the aeroplane passes over the beacon in its approach its position is signalled to the pilot by the sound of a buzzer or the flash of a lamp on the instrument panel.

Outflow: The increase in air velocity behind an airscrew produced by its rotation.

Overhang: (1) The extent to which the wingtip of one or two superimposed planes projects beyond the tip of the other. (2) The distance from the outer point of support to the tip of an aerofoil.

Overstrain: The condition of a structural material which has been stressed past the yield point. Materials can be annealed - given a form of heat treatment - to remove the effects of overstrain.

Oxidation: The action of combining any substance with oxygen. Oxidation may be rapid, as in an explosion; of moderate speed, as in the burning of solid fuels; or slow, as in the rusting of metals.

Oxygen Equipment: Oxygen equipment is provided in all modern Service aircraft. The installation includes cylinders containing oxygen under pressure, regulators to control the supply pressure, flow meters and a breathing mask for each member of the crew.

Oxygen-mask Microphone: A microphone for radio transmission and intercommunication between aircrew members fitted to an oxygen breathing mask