a) In the classic sense, thixotropy refers to a material that exhibits reversible sol-gel-sol-… behavior. For example, ‘chemically modified’ bentonite drilling muds used by the petroleum industry are thixotropic. The property prevents the granular material (“cuttings”) produced during drilling from settling out when drilling is stopped (briefly, or for substantial periods), thus preventing the drill rod from seizing.

b) The ‘modern viscometric’ meaning refers to the increase in shear resistance when the shear rate is increased, and the decrease when the shear rate is reduced. Most thoroughly dispersed clay materials above some relatively low concentration exhibit this phenomenon.
Cf., dilatancy, Newtonian fluid, quick clay, rheopexy