Thixotropy is the property of some non-newtonian pseudoplastic fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. However, this is not a universal definition; the term is sometimes applied to pseudoplastic fluids without a viscosity/time component. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

It is important to note the distinction between a thixotropic fluid and a shear thinning fluid. The former displays a decrease in viscosity over time at a constant shear rate, while the latter displays decreasing viscosity with increasing shear rate. Fluids which exhibit the opposite property, in which constant shear stress for a time, such as shaking or mixing, causes an increase in viscosity or even solidification, are called rheopectic, sometimes called anti-thixotropic, and are much less common.