Eric’s talk at SciPy ’07 involved an example graph showing various properties of a thixotropic fluid. Thixotropy is a time-dependent decrease in the thickness of a fluid under a shearing force. The time dependence distinguishes a thixotropic fluid from a shear-thinning fluid; the latter recovers its original viscosity as soon as the shearing force is removed, as illustrated in this graph (different from Eric’s graph).
The most notable common thixotropic fluid is tomato ketchup. Thixotropy is why you can still pour the ketchup after you stop shaking the bottle. If ketchup were merely shear-thinning, you would have to keep shaking the bottle while pouring, which would be even messier. More likely, shear-thinning ketchup wouldn’t come in glass bottles. Most ketchup comes in squeezable plastic bottles these days, but if you encounter a glass ketchup bottle in a restaurant, there are several techniques for getting it to pour.
The word “thixotropy” derives from the Greek thixis, a touch, and tropos, to turn or change. While the -tropy suffix is common in scientific terminology, thixo- is much less so. The root appears more often as thigmo-. The only other thixo- form is thixophobia, irrational fear of touching or being touched.