The simplest definition of Occam's Razor is "Don't make unnecessarily complicated assumptions". It can be used as a philosophical way of sorting the simple theories from the complicated ones. When scientists select theories, they don't just use the criterion of agreement or disagreement with observations. They also have aesthetic principles, and a desire for an elegant, universal theory. They use these aesthetic principles to remove the cloud of trivially competing theories that necessarily surround every theory. Occam's razor is a working rule of thumb, not the ultimate answer.
Cohen J., and I. Stewart (1994). The Collapse of Chaos. New York: Viking Press.
Contributed by: L.A. Keple, October 6, 1995