1. n. 
See: aniline point test, attapulgite, brine, bypassed mud, calcium chloride, calcium test, Chenevert method, chloride test, coalescence, colloid, creaming, diesel-oil mud, dispersion, drilling fluid, electrical stability test, fatty acid, fatty-acid soap, flash point, hectorite, high-pressure, high-temperature viscometer, hydrophile-lipophile balance number, hygrometer, inhibit, inhibitive mud, interfacial tension, invert-emulsion oil mud, lignite, oil-base mud, oil-mud emulsifier, oil/water ratio, organophilic, organophilic lignite, pour point, rheology modifier, shale, sheen test, slurry, surface tension, suspended solids, synthetic-base mud, synthetic/water ratio, water-in-oil emulsion
2. n. 
More generally, a mud system that has any type of nonaqueous fluid as the external phase. This definition would include the newer variety of oil muds that are more exactly defined as synthetic-base muds. Synthetic mud is analogous to oil mud.