Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral, synthetic or vegetable oil.

A common feature of greases is that they possess high initial viscosities, which upon the application of shear, drop to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. This change in viscosity is called shear thinning. The term grease is also sometimes used to describe lubricating materials that are simply soft solids or high viscosity liquids, but these materials do not exhibit the shear-thinning properties characteristic of greases meeting the narrower definition. For example, petroleum jellies such as Vaseline are not generally classified as greases.

Greases are applied to mechanisms that can be lubricated only infrequently and where a lubricating oil would not stay in position. They also act as sealants to prevent the ingress of water and incompressible materials. Grease-lubricated bearings have greater frictional characteristics because of their high viscosities.

A true grease consists of an oil or other fluid lubricant that is mixed with a thickener, typically a soap, to form a solid or semisolid. Greases are usually shear-thinning or pseudo-plastic fluids, which means that the viscosity of the fluid is reduced under shear stress. After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base lubricant, such as mineral oil. This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic. A few greases are rheotropic, meaning they become more viscous when worked. Grease is often applied using a grease gun, which applies the grease to the part being lubricated under pressure, forcing the solid grease into the spaces in the part.

Soaps are the most common emulsifying agent used, and the selection of the type of soap is determined by the application. Soaps include calcium stearate, sodium stearate, lithium stearate, as well as mixtures of these components. Fatty acids derivatives other than stearates are also used, especially lithium 12-hydroxystearate. The nature of the soaps influences the temperature resistance (relating to the viscosity), water resistance, and chemical stability of the resulting grease. Calcium sulphonates and polyureas are increasingly common grease thickeners not based on metallic soaps.

Powdered solids may also be used as thickeners, especially as absorbent clays like bentonite. Fatty oil-based greases have also been prepared with other thickeners, such as tar, graphite, or mica, which also increase the durability of the grease. Silicone greases are generally thickened with silica.

Lithium-based greases are the most commonly used; sodium- and lithium-based greases have higher melting points (dropping points) than calcium-based greases but are not as resistant to the action of water. Lithium-based grease has a dropping point at 190 to 220 °C (374 to 428 °F). However, the maximum usable temperature for lithium-based grease is 120 °C.

The amount of grease in a sample can be determined in a laboratory by extraction with a solvent followed by e.g. gravimetric determination.