Shear thinning is a phenomenon characteristic of some non-Newtonian fluids in which the fluid viscosity decreases with increasing shear stress. Shear thickening is the opposite phenomenon. (By contrast to both, viscosity in Newtonian fluids is by definition independent of the forces exerted on the fluid.) Fluids that exhibit shear thinning are sometimes called pseudoplastics and are typically complex fluids such as blood, motor oil, ketchup, and even whipped cream, though simple fluids can also exhibit the behavior near their critical point (ex: xenon in Ref. 1).
What causes the phenomenon is still not fully understood, but it is generally considered to be the result of microscale structural rearrangements within the fluid, such as a change in the way red blood cells move about in blood (Keyword Ref. 1) or the organization of microspheres into hexagonally packed structures that slide over each other more easily (i.e. exhibit lower viscosity) than at lower shears where randomly organized particles collide frequently (Ref. 2).
Shear thinning proves useful in many applications, from lubricating fast-moving engine parts to making an otherwise stiff biocompatible hydrogel injectable (Ref. 4).