[edit] Medical profession
From the nineteenth century onward, "doctor" has been commonly used as a synonym for "physician" in Anglophone and many other countries; this term is commonly used as a title of address for physicians, whether or not they hold a doctorate. The primary medical qualification in the UK and in many Commonwealth countries are the 'Bachelor of Medicine and Bachelor of Surgery' degrees (MB BS, MB BCh, MB ChB, BM BCh or MB BChir, depending on the University granting the award). In the UK the title "Dr" is officially conferred by the General Medical Council to graduates whose names are included on the list of 'registered medical practitioners', a prerequisite to medical practice in the UK. After qualification, medical practitioners may read for the postgraduate research degree of 'Doctor of Medicine'.
In the United States, however, the degrees Medicinae Doctor (Latin, "Doctor of Medicine") and Doctor of Osteopathy (D.O.) are the qualifying degrees. There is no prefatory undergraduate medical degree, but students theoretically may enter an M.D. or D.O program with a baccalaureate in any subject. Practically, however, most "Pre-Med" students take their undergraduate degrees with a major/minor concentration in biology and/or chemistry. American medical schools expect a baccalaureate degree to have been completed prior to admission to an M.D. or D.O. program. However, many medical schools also allow for some students to matriculate prior to completing their baccalaureate degree provided that they have successfully completed at least three years of undergraduate course work.
In American and Canadian parlance, "doctor" is most often used for all types of physicians and surgeons, including internists, pediatricians, psychiatrists, gynecologists, and all other surgical and nonsurgical specialists who hold M.D. or D.O. degrees. The term may also apply to other fields of healthcare, to which the degrees 'M.D.' and 'D.O.' are inapplicable, including doctors of Chiropractic (D.C.), doctors of Naturopathic Medicine (N.D.), doctors of Pharmacy (Pharm.D.), doctors of Physical Therapy (D.P.T.), doctors of Podiatric Medicine (D.P.M.), doctors of Optometry (O.D.), doctors of Audiology (Au.D.), doctors of Dental Surgery/Dental Medicine (D.D.S./D.M.D.) and so on.
Note, though, that this is not the case in Germany, where a doctoral degree, and the title "Doktor" is only awarded to medical students if they complete a separate research thesis during their medical studies. The status of being a licenced physician (an "Arzt") is quite separate from this and comes from passing Medical Board Exams set by one of the German states (Bundesländer). A German physician who is licenced to practise medicine (passed his/her medical exams) but did not complete a research thesis would consider it inaccurate and even improper to style him/herself "Dr". They would typically write their qualifications in the form (in English): "State Med Exam (state)(year)" rather than the "MD" MB BS" or "MB ChB" familiar in Anglophone countries.