Anisotropy
Anisotropy is the quality of having a different distance measure in different directions. Shapes that are considered anisotropic include oblong and disc like structures.
In estimation, anisotropy refers to the shape of the search ellipsoid. It is not equidistant in the X, Y, and Z directions. The search ellipsoid could be the same distance from the centroid in two directions but not the third, or it could be different distances from the centroid in all directions. This means that the distribution of the samples used in the estimation is weighted higher in the longer directions.