anisotropic shading - shading that stretches reflections and highlights in a direction that runs perpendicular to the grain or grooves in a surface. Contrast anisotropic shading with the more common isotropic shading (shown on the left side above), which scatter light evenly in all directions.  For an example of how surface grain shapes highlights, study this photograph of human hair; because there is more curvature running around these hairs than running along the hairs, hair in this photograph is shown scattering light anisotropically, producing highlights that run perpendicular to the hair direction.  Brushed steel, phonograph records, compact discs, and wooden objects also exhibit anisotropic shading with highlights running perpendicular to the grooves or grain.

Source: Digital Lighting & Rendering by Jeremy Birn, definition and 3D images adapted from the book by permission.  This page © 2001 by Jeremy Birn.