Idler – a wheel, gear or roller inserted into a drive train primarily for the function of guiding, supporting or tensioning a portion of the drive train.

Immiscible -- incapable of being mixed without separation of phases. Water and petroleum oil are immiscible under most conditions, although they can be made miscible with the addition of an emulsifier.

In-line filter -- a filter assembly in which the filter element is mounted along the same orientation as the filter inlet and outlet.

Indicator -- a device which provides external evidence of sensed phenomena.

Indicator, differential pressure -- an indicator that signals the difference in pressure between two points, typically between the upstream and downstream sides of a filter element.

Influent -- the material entering a system.

Infrared spectra -- a graph of infrared energy absorbed at various frequencies in the additive region of the infrared spectrum. The current sample, the reference oil and the previous samples are usually compared.

Ingested contaminants -- environmental contaminant that enters a system due to the action of the system or machine.

Inhibitor -- any substance that slows or prevents such chemical reactions as corrosion or oxidation.

Insolubles -- residues of carbon or agglomerates of carbon and other material such as spent additives or oxidation by-products.

Interfacial tension (IFT) -- the energy per unit area present at the boundary of two immiscible liquids, usually expressed in dynes/cm.

ISO Solid Contaminant Code -- a index number code assigned on the basis of the number of particles per unit volume, allowing quick assessment of contamination.

ISO viscosity grade -- a number indicating the nominal viscosity of an industrial fluid lubricant at 40°C (104°F) as defined by ISO Standard 3448. For example, an unused ISO 68 grade product would be expected to show a viscosity at 40°C of 68 cSt, plus or minus 10% (6.8 cSt). This pattern is maintained throughout the ISO viscosity grading system.