Isotropy is uniformity in all orientations. In microfabrication, it means that the process proceeds at the same rate regardless of direction. For instance, most chemical etches are isotropic: they will etch in all directions equally.
The opposite, anisotropy refers to a situation where the process proceeds more favorably in one direction over another. A highly anisotropic etch process (such as reactive ion etching will proceed highly vertically and have minimal undercut.
|Isotropy||Red: masking layer; yellow: layer to be removed