From The Collaborative International Dictionary of English v.0.48:
Dr \Dr.\ n.
abbreviation for doctor, a title accorded to a person who
holds a doctorate degree from an academic institution, such
as a Ph.D. degree or M.D. degree. [abbrev.]
Syn: doctor.
[WordNet 1.5]
2. a licensed doctor of medicine.
Syn: doctor, doc, physician, MD, medico.
[WordNet 1.5]