ordinal number. Ordinal numbers indicate position in a series or order such as first, second, and third. They tell us the order in which something occurs. The first day, the second order, or the third time are examples of the use of ordinal numbers.
ordinate. The vertical line (vertical coordinate, or Y-axis) drawn on a graph. Variable quantities are measured by the scale to which the ordinate is drawn. The horizontal coordinate (X-axis) on a graph is called the abscissa.

organic brake linings. Brake linings for single-disk brakes, made of a high-friction organic material reinforced with brass or copper wool. Asbestos was used in the past, but because of its harmful effect on those who work with it, organic materials have been developed by most brake manufacturers to replace asbestos.
organic fabric (aircraft fabric). Natural-fiber fabrics used for covering aircraft structures. Long-staple cotton and linen are the two most widely used organic fabrics.
OR gate (logic gate). A logic gate having two or more inputs and one output. An OR gate has a logic one on its output when there is a logic one on any one of its inputs. There are two kinds of OR gates: INCLUSIVE OR, and EXCLUSIVE OR. An INCLUSIVE OR gate has a logic one on its output any time one or more of its inputs are at logic one. An EXCLUSIVE OR gate has a logic one on its output when one and only one of its inputs is at logic one.

orifice. A hole. When the word orifice is used, the hole is considered to have special characteristics. The orifice in a carburetor jet, for example, has a specific diameter, length, and approach and departure angles.

orifice check valve (fluid power system component). A special check valve used in a hydraulic or pneumatic system, to allow full flow of fluid in one direction but a restricted flow in the opposite direction.