Deaerator – a separator that removes air from the system fluid.

Defect – an imperfection in a material that contributes significantly to failure or limited serviceability.

Degas -- removing air from a liquid, usually by ultrasonic and/or vacuum methods.

Degradation -- the progressive reduction of performance, usually ending in failure of a machine or lubricant.

Dehydrator -- a separator that removes water from the system fluid.

Delamination -- a complex wear process where a machine surface is peeled away or otherwise removed by forces of another surface acting on it in a sliding motion.

Demulsibility -- the ability of a fluid that is insoluble in water to separate from water with which it may be mixed in the form of an emulsion.

Density -- the mass of a unit volume of a substance. Its numerical value varies with the units used.

Deposit -- process of a solid residue accumulating on a surface. For example, oil-insoluble materials that result from oxidation and decomposition of lube oil and contamination from external sources and engine blow-by are deposited on machine or engine parts. Other examples are sludge, varnish, lacquer and carbon.

Depth filter -- a filter medium that retains contaminants primarily within the winding deep structure of the filter.

Desorption -- opposite of absorption or adsorption. In filtration, it relates to the downstream release of particles previously retained by the filter.

Detection limit -- practically defined as the point where the signal level decreases to less than two to three times the noise level.

Detergent -- in lubrication, either an additive or a compounded lubricant having the property of keeping insoluble matter in suspension thus preventing its deposition where it would be harmful. A detergent may also redisperse deposits already formed.

Dielectric Strength -- a measure of the ability of an insulating material to withstand electric stress (voltage) without failure. Fluids with high dielectric strength (usually expressed in volts or kilovolts) are good electrical insulators.

Dieseling – the continued running of a spark-ignited engine after the ignition is turned off. There are two basic causes of dieseling:

surface ignition, where combustion chamber surfaces remain hot enough to ignite fuel after the spark is terminated;

compression ignition, where the conditions of temperature, pressure, fuel composition and engine idle speed allow ignition to continue.

Differential pressure indicator -- an indicator that signals the difference in pressure between any two points of a system or a component.

Disposable -- a filter element intended to be discarded and replaced after one service cycle.

Dissolved gases -- those gases that enter into chemical solution with a fluid, as distinguished from free or entrained gases (mechanically mixed but not chemically bound).

Distillation -- process of driving gas or vapor from liquids or solids by heating, then condensing the vapor for the purposes of separation, purification, or measurement.

Drum -- a container with a capacity of 55 U.S. gallons.

Dry-film lubricant -- solid material deliberately deposited between two moving surfaces to prevent metal-to-metal contact, thus reducing friction and wear. Such materials are especially useful in the region of boundary lubrication, and for lubrication under special conditions of extremely high or low temperature where usual lubricants are inadequate. Some examples are graphite or molybdenum disulfide.

Duplex filter -- an assembly of two filters with valving for selection of either or both filters.