Vacuum grease is a lubricant

Lubricant

A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...

 with low volatility and is used for applications in low pressure environments. Lubricants with higher volatility would evaporate, causing two problems:

• They would not be present to provide lubrication.
• They would make lowering the pressure below their vapor pressure difficult.

As well as a lubricant, vacuum grease is also used as a sealant for joints in vacuum systems. This is usually limited to soft vacuums, as ultra high vacuum

Ultra high vacuum

Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

 or high temperatures may give problems with the grease outgassing

Outgassing

Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...

. Grease is most commonly used with glass vacuum systems. All metal systems usually use knife-edge seals in soft metals instead. Where O ring seals are used, these should not be greased (in static seals at least) as it can cause the O rings to become permanently distorted when compressed., p. 115

In electronics manufacturing processes, vacuum grease is often used to prevent corrosion.