A lubricant (colloquially, lube) is a typically liquid substance introduced between two moving surfaces to reduce the friction and wear between them. Worldwide, more than 50 million tonnes of lubricants are consumed annually. Automotive engine lubricants comprise approximately 50% and hydraulic fluids with transmission oils 30% of lubricants volume-wise. In developed nations, lubricants contribute to 1/4 of total pollution released to environment.

Typically lubricants contain some 90% of base oil (most often petroleum fractions, called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, resistance to corrosion and aging, etc.. Sodium and lithium based additives are used in automotive greases to stabilise the grease against the high temperatures. This is particularly important in the grease used to pack wheel bearings, and most especially those used with disc braking systems.

Non-liquid lubricants include grease, powders (dry graphite, PTFE, Molybdenum disulfide, etc.), Teflon tape used in plumbing, air cushion and others.

In addition to automotive and industrial applications, lubricants are used for many other purposes, including personal hygiene, e.g. K-Y jelly, often used as a sexual lubricant, bio-medical applications (e.g. lubricants for artificial joins) and others.

An alternative way to reduce friction and wear is to use bearings such as ball bearings, roller bearings or air bearings. Some ball and roller bearings are also lubricated, and some are not, and this is extremely important. Any bearing designed for lubrication will not surprisingly fail if it is not lubricated properly. What is less obvious is that a bearing designed to operate without lubrication will often fail if lubricated.

A spectacular example of this is the result of lubricating a roller bearing with molybdenum disulphide grease. This so effectively reduces the friction between the rollers and the inner and outer that the rollers do not roll, but instead slip, wearing two slight flats on the roller. As soon as this process has begun the flats rapidly develop, and the bearing fails.