(Link) It’s defined as the “relative motion between adjacent layers of moving liquid”. As above, “Shearing occurs whenever the fluid is physically moved or distributed, as in pouring, spreading, spraying, mixing, etc.” A high shear mixer – like an immersion or stick blender – will brings those layers together quicker than something that’s low shear, like a hand mixer. High shear mixers spin quicker in the middle than the outsides.