One of the popular lubricants used to prevent friction heat in bearings.
Usually a compound of mineral oil with a soap. Chemicals can be added to the compound to improve the physical properties of the grease.
Oil is the preferred method of lubricating a bearing, but you have to be able to maintain an oil level. Grease is often the only alternative in vertical pump applications, or in those designs where the radial and thrust bearings have different diameters.
Grease can be compounded for just about any lubricating application.
The biggest problem we have with grease lubrication in ball bearings is the frequency of over lubrication problems.