is a substance (usually a liquid) introduced between two moving surfaces to reduce the friction and wear between them. A lubricant provides a protective film which allows for two touching surfaces to be separated, thus lessening the friction between them.  Typically lubricants contain 90% base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicone, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc.  Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, etc.), teflon tape used in plumbing, air cushion and others. Another approach to reducing friction and wear is to use bearings such as ball bearings, roller bearings or air bearings or to use sound, in the case of acoustic lubrication.  Lubricants such as 2-cycle oil are also added to some fuels. Sulfur impurities in fuels also provide some lubrication properties, which has to be taken in account when switching to a low-sulfur diesel; bio-diesel is a popular diesel fuel additive providing additional lubricity.  In addition to automotive and industrial applications, lubricants are used for many other purposes, including K-Y Jelly, often used as a personal lubricant, bio-medical applications (e.g. lubricants for artificial joints) and others.

Also see aluminum rolling oil, anti-seize compound, automatic transmission fluid, block grease, chain oil, concrete form coating, compounded oil, coning oil, cutting fluid, cylinder oil, drawing compound, engine oil, fire-resistant fluid, fire-resistant grease, gear oil (automotive), gear oil (industrial), gear shield, graphite, grease, hydraulic fluid, industrial lubricant, ink release agent, launching lubricant, metalworking lubricant, mineral oil, mineral seal oil, mold lubricant, molybdenum disulfide, multi-grade oil, multi-purpose grease, paper machine oil, rail-flange grease, refrigeration oil, ring oil, rock drill lubricant, rolling oil, single-grade oil, soluble oil, straight mineral oil, synthetic turbo oil, thread compound, torque fluid, turbine oil, way lubricant.