The Collaborative International Dictionary of English v.0.48:

PhD \PhD\, PhD \Ph.D.\n. [Abbrfev. from L., Philosophae Doctor.]
   A scholastic degree awarded for study beyond college; the
   Doctorate degree in Philosophy. In America it is usually
   based on at least 3 years graduate study and a dissertation;
   the doctorate is the highest degree awarded by a graduate
   school, and is awarded in other fields, such as Doctor of
   Arts. The Ph.D. is broadly applicable, as in most fields of
   science and technology; -- the term philosophy being derived
   from an earlier label for experimental science as Natural
   Philosophy.

   Syn: Doctor of Philosophy.
        [WordNet 1.5 + PJC]

WordNet (r) 3.0 (2006):

PhD
    n 1: a doctorate usually based on at least 3 years graduate
         study and a dissertation; the highest degree awarded
         graduate study [syn: Ph.D., PhD]