http://www.newadvent.org/cathen/05407a.htm gives the following:
"Primarily, and in its psychological application, the term signifies the theory that the phenomena of consciousness are simply the product of sensuous experience, i.e. of sensations variously associated and arranged. It is thus distinguished from Nativism or Innatism. Secondarily, and in its logical (epistemological) usage, it designates the theory that all human knowledge is derived exclusively from experience, the latter term meaning, either explicitly or implicitly, external sense-percepts and internal representations and inferences exclusive of any superorganic (immaterial) intellectual factor. In this connection it is opposed to Intellectualism, Rationalism, Apriorism. The two usages evidently designate but two inseparable aspects of one and the same theory the epistemological being the application of the psychological to the problem of knowledge.
Empiricism appears in the history of philosophy in three principal forms: (1) Materialism, (2) Sensism, and (3) Positivism."
The definition as stated needs to take on some of these --AvengingAngel 17:48, 26 April 2007 (EDT)