The characteristic of a fluid, such as a drilling mud, to form a gelled structure over time when not subject to shearing and then to liquefy when agitated. The viscosity of a thixotropic fluid changes with time under constant shear rate until reaching equilibrium. Most drilling muds exhibit thixotropy, which is necessary for fast drilling, efficient cuttings lifting and to support weighting material when mud flow stops. Gel strength measured at various time intervals indicates the relative thixotropy of a mud.

Thixotropy is sometimes desirable to provide resistance to flowing, such as to avoid or reduce losses or flow into a weak formation.