A lubricant (sometimes referred to as a "Lube") is a substance (often a liquid) introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear. They also have the function of dissolving foreign particles. Petroleum-based lubricants like Vaseline tend to dissolve petroleum products such as rubber and plastic, while water-based lubricants will dissolve polar chemicals. The lubricant must be replaced when it has dissolved to saturation, because the inability to dissolve additional debris allows abrasive particles to scrape against or become lodged in the working surfaces. Lubricants which dissolve working surfaces (such as Vaseline with rubber) defeat their purpose by corroding the smooth surfaces by their own dissolving power, thus compromising structural integrity, surface smoothness, and system-wide contamination.

One of the single largest applications for lubricants, in the form of motor oil, is to protect the internal combustion engines in motor vehicles and powered equipment.

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.

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; biodiesel is a popular diesel fuel additive providing additional lubricity.

Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, tungsten disulfide, etc.), teflon tape used in plumbing, air cushion and others. Dry lubricants such as graphite, molybdenum disulfide and tungsten disulfide also offer lubrication at temperatures (up to 350 °C) higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems, however, practical use is still many years away due to their physically unstable nature.

Another approach to reducing friction and wear is to use bearings such as ball bearings, roller bearings or air bearings, which in turn require internal lubrication themselves, or to use sound, in the case of acoustic lubrication.

In addition to automotive and industrial applications, lubricants are used for many other purposes, including as a personal lubricant, bio-medical applications (e.g. lubricants for artificial joints) and others.