Lubrication involves introducing many substances between sliding surfaces to reduce wear and friction. Since the evolution of lubricants, nature has been utilizing them, with crude oil becoming the primary source for lubricating oils. By developing a wide range of products to cater to the diverse lubrication needs of vehicles, aircraft, internal combustion engines, turbojet engines, and numerous power mechanisms, the natural lubricating capabilities of crude oil have steadily improved.

The improvement of petroleum lubricants has, in turn, increased the speed and capacity of industries and other machinery. The separation of liquid films from sliding surfaces leads to this lubrication. Liquids can be intentionally introduced as oil in the main bearings of vehicles, or unintentionally as water between smooth rubber tires and wet surfaces. Although liquids are typically liquids, they can also be gases. Air is a commonly used gas.

To maintain separation, the pressure within the lubricating film must be balanced against the load on the sliding surface. If the pressure of the lubricating film is supplied by an external source, the system is known as hydrostatic lubrication. However, if the pressure between surfaces is due to the shape and movement of the surfaces themselves, the system is hydrodynamic lubrication. Lubrication depends on the viscosity characteristics of the lubricant. A state between unlubricated sliding and fluid film lubrication is referred to as boundary lubrication, which is also defined as the lubrication state where the friction between surfaces is determined by surface and lubricant characteristics (excluding viscosity). Boundary lubrication is an important part of the lubrication phenomenon and typically occurs during the start-up and shutdown processes of machinery.