1. n. 
Any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Synonymous with "drilling mud" in general usage, although some prefer to reserve the term "drilling fluid" for more sophisticated and well-defined "muds." Classifications of drilling fluids has been attempted in many ways, often producing more confusion than insight. One classification scheme, given here, is based only on the mud composition by singling out the component that clearly defines the function and performance of the fluid:
Each category has a variety of subcategories that overlap each other considerably.
For more details, see The Defining Series: Drilling Fluid Basics.
Synonyms: drilling mud
See: core, emulsion, emulsion mud, formate, formation damage, glycol, guar gum, gyp mud, heavy metal, hydrometer, inhibitive mud, lime mud, oil mud, saltwater mud, suspended solids, synthetic-base mud, water-based drilling fluid