Hardness -- the resistance of a substance to surface abrasion.

Heat exchanger -- a device which transfers heat through a conducting wall from one fluid to another.

Housing -- a ported enclosure which directs the flow of fluid through the filter element.

Hydraulic Fluid -- fluid serving as the power transmission medium in a hydraulic system, with specific properties tailored to the fluid's use in this application.

Hydraulics -- engineering science pertaining to liquid pressure and flow.

Hydrocarbon -- a compound containing only hydrogen and carbon. The simplest hydrocarbons are gases at ordinary temperatures; but with increasing molecular weight, they change to the liquid form and, finally, to the solid state. They form the principal constituents of petroleum.

Hydrodynamic lubrication -- a system of lubrication in which the shape and relative motion of the moving surfaces causes the formation of a fluid film having sufficient pressure to separate the surfaces.

Hydrofinishing -- a process for treating raw extracted base stocks with hydrogen to saturate them for improved stability.

Hydrolysis -- breakdown process that occurs in anhydrous hydraulic fluids as a result of heat, water, and metal catalysts (iron, steel, copper, etc.)

Hydrolytic stability -- ability of additives and certain synthetic lubricants to resist chemical decomposition (hydrolysis) in the presence of water.

Hydrometer -- an instrument for determining either the specific gravity of a liquid or the API gravity.

Hydrostatic lubrication -- a system of lubrication in which the lubricant is supplied under sufficient external pressure to separate the opposing surfaces by a fluid film.

Hypoid gear lubricant -- a gear lubricant especially designed for hypoid gear types, as in the differential of an automobile. Hypoid gear tooth contact involves a combination of radial and sideways sliding which is intermediate between the worm type and the spiral bevel type gear, requiring additional EP (extreme pressure) additization for proper function.