1. n. 
A term that is generally synonymous with drilling fluid and that encompasses most fluids used in hydrocarbon drilling operations, especially fluids that contain significant amounts of suspended solids, emulsified water or oil. Mud includes all types of water-base, oil-base and synthetic-base drilling fluids. Drill-in, completion and workover fluids are sometimes called muds, although a fluid that is essentially free of solids is not strictly considered mud.
Synonyms: drilling mud
See: conventional mud, emulsion mud, invert-emulsion oil mud, lubricant, mud weight, mudding up, native-solids mud, oil mud, pill, reserve-mud pit, returns, slug, slurry, spud mud, suspended solids, unweighted mud, weighted mud