Physician is a term which generally means a doctor trained to diagnose and treat human diseases and not anatomically limited. Thus by this definition a Medical doctor, Osteopath (in the US) and a Chiropractor are all physicians.
The term is a word regulated by individual State laws, not by professional associations; in the same way that "bank" is a regulated term. In the majority of the US States, a Doctor of Chiropractic is defined as a physician. In addition, the US Federal government, under the laws for Medicare, define any doctor receiving medicare benefits as a physician. Notable exceptions to this rule include California and New York. Of course, when it must be remembered that e.g. New York was the 48th State to grant licensing to Chiropractors, and at that time, fully one third of the membership of the AMA was in New York.