octahedron. A solid form that has eight plane (flat) surfaces.
octal number system. A system of numbers based on eight units (zero through seven). Octal numbers are used in digital electronics to make long strings of zeros and ones more manageable. One octal digit replaces
three binary digits. 
Decimal  Octal  Binary 
0  0  000 
1  1  001 
2  2  010 
3  3  011 
4  4  100 
5  5  101 
6  6  110 
7  7  111 

octane rating (reciprocating engine fuel rating). The system used to rate the ability of a reciprocating-engine fuel to resist detonation. The higher the octane number, the more resistant the fuel is to detonation.
The fuel being rated is run in a test engine whose compression ratio can be varied. This ratio is changed until the fuel detonates, or explodes inside the cylinder, rather than burning as it should. The engine is left with the compression ratio that caused the fuel to detonate, and a fuel composed of a mixture of iso-octane, a fuel that resists detonation, and heptane, a fuel prone to detonate, is fed to the engine.
The ratio of the two fuels is varied until the engine detonates as it did with the fuel being rated. When the performance with the two fuels is matched, the ratio of the octane to heptane is noted, and the fuel is rated with the percentage of octane used. If a mixture of 80% octane and 20% heptane causes the engine to perform as it did with the fuel being rated, the fuel is given an octane number of 80. At one time aviation gasoline was given a dual rating such as 80/87. The first number represented its antidetonation rating with a lean, cruise fuel-air mixture, and the higher number, its rating with a rich, takeoff mixture. Fuel with antidetonation characteristics better than that of iso-octane is rated in performance numbers. See performance numbers.
octave. An interval of audible vibrations spanning eight full tones above or below another. A tone, in this regard, is a sound with a distinct pitch.
odd harmonics. The odd multiples of a fundamental frequency. For example, frequencies that are three, five, seven, and nine times that of a fundamental, or original, frequency are odd harmonics of the frequency.
odometer. The portion of an automobile speedometer that indicates the distance traveled.
oersted. A unit of magnetic intensity in the centimeter-gram-second system of measurement. One oersted is the amount of magnetic intensity equal to one gilbert per square centimeter, and this is 79.577 ampere-turns per meter.
off course (air traffic control). A term used to describe a situation where an aircraft has reported a position fix or is observed on radar at a point not on the ATC-approved route of flight.