1. n. [Drilling Fluids]

A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature.

See related terms: colloidal solids,  gel strength,  oil mud,  progressive gels,  shear-strength measurement test,  yield point