diluent. A material used to thin, or dilute, a concentrated material without changing its basic characteristics. A weak solution contains a large amount of diluent.
Paint thinner is a diluent used to make paint less viscous so it can be sprayed or brushed more easily. The diluent evaporates after the paint is applied.
diluter-demand oxygen system. A popular type of breathing oxygen system in which oxygen is metered to the mask where it is diluted with cabin air by an airflow-metering aneroid assembly which regulates the amount of air allowed to dilute the oxygen on the basis of cabin altitude.
As the aircraft goes to higher altitude, the amount of cabin air metered into the oxygen is automatically decreased until at about 34,000 feet, the cabin air is entirely shut off and the regulator sends 100% oxygen to the mask.
The oxygen and air flow to the mask only when the wearer inhales, but the regulators have an emergency position which allows the pilot to manually bypass the demand function and sends a continuous flow of 100% oxygen to the mask.
dimension. A measurement used to describe the size of an object. The basic dimensions of an object are its L (length), W (width), and H (height).
dimensional inspection. One step in the overhaul of a piece of equipment. In a dimensional inspection, all the moving parts are measured with precision instruments, such as micrometer calipers, to determine if their dimensions are within the tolerances allowed by the manufacturer for the part to be serviceable.
dimension line (mechanical drawings). A light solid line on a machine drawing. A dimension line is drawn between extension lines that indicate the beginning and the end of a dimension.
A dimension line has an arrowhead at each end and a break in its middle where the dimension and the tolerance are written.

dimmer (electric lights). A circuit or device used to decrease the intensity of an electric light.
A dimmer may decrease the voltage supplied to the light by adding resistance, as is done with a rheostat. Or it may shorten the time electrons flow in each cycle of alternating current as is done with an SCR, a silicon controlled rectifier.
dimpling (sheet-metalworking procedure). The treatment of a hole in a piece of sheet metal for the installation of a countersunk rivet or screw.