Fresnel Effect (pronounced "fre-nel," the "s" is silent) - the observation that the amount of reflectance you see on a surface depends on the viewing angle.  As shown in the renders above, if you look straight down from above at a pool of water, you will not see very much reflected light on the surface of the pool, and can see down through the surface to the bottom of the pool.  At a glancing angle (looking with your eye level with the water, from the edge of the water surface), you will see much more specularity and reflections on the water surface, and might not be able to see what's under the water.

Fresnel Shaders in general allow reflection, specularity, and other attributes to vary according to the viewing angle of a 3D surface.  A Fresnel shader will let you specify a specular color for parts of a surface directly facing the camera, and another specular color to be seen on parts of a surface that are perpendicular to the camera. In addition to the usual specular highlights and reflectivity, parameters such as the color or transparency can also be controlled by some Fresnel shaders.

The Fresnel Effect was first documented by the French physicist Augustin-Jean Fresnel (1788-1827), who advanced the wave theory of light through a study of how light was transmitted and propagated by different objects. In addition to the Fresnel Effect, Augustin-Jean Fresnel is also known as the inventor of the Fresnel Lens, which is still used in lighthouses around the world, and is used in film production on a Fresnel Light.

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