Greases are lubricants which have a semi-liquid to solid consistency. They are not simply very viscous oils but are actually oils mixed with other ingredients to give a gel like consistency.
When they are used as lubricants, greases behave like oils in many ways. They reduce friction by providing a film which separates moving surfaces and, as we will see later on, it is actually oil that carries out this lubrication rather than the grease itself. However, unlike oils, greases have the advantage that they tend to stay where they are put.
They are less likely than oils to leak out of a machine and less likely to flow away from the surface that they lubricate under the effect of gravity or centrifugal forces. At the same time, and this can be significant in many airframe applications, greases can form an effective seal against moisture and solid contaminants.
These properties mean that greases can often act as effective and economical lubricants, especially in mechanisms which are hard to reach or require infrequent lubrication.