Exactly what does doctor mean, and who can call himself or herself a doctor, and who can’t? A discussion of the term and its origins and parameters follows.
Doctor derives from the Latin verb docere, meaning “teach”; it is also the origin of docent, originally an adjective but now almost invariably used as a noun to refer to a museum or art gallery tour guide (although it also denotes a university instructor who is not a professor), and docile, which originally meant “easily taught” but by extension now refers primarily to obedience and submissiveness. Document, which referred originally to a lesson but now denotes any written proof or evidence, or, by extension, any written content, is also related, as is the noun doctrine, meaning “teaching.” (Someone with strict, narrow beliefs, or such beliefs themselves, is referred to as doctrinaire.)
The original doctors were what we now call professors or teachers; those who taught anatomy, medicine, and other health-related disciplines inspired the word to be used as a title for a specific class of professionals who practice medicine. However, it still also denotes someone who has obtained a research doctorate and is entitled to use the initials PhD (often styled in the older form Ph.D. as well), which stands for “doctor of philosophy.” (Originally, philosophy, meaning “love of wisdom,” referred to a wide array of what we would now call the arts and sciences and only later pertained strictly to the study of knowledge, truth, and life.)
Those who hold a doctorate and work in the medical field but do not have a doctor of medicine (MD) or doctor of osteopathy (OD) degree are generally required to advise patients that they do not hold one of these degrees. (An osteopath—the word is from a Greek term meaning “sensitive to or responding to bone”—is in the United States qualified to practice medicine just like someone with an MD degree, but in other countries such a practitioner is more limited in scope.)
Many other medical practitioners, such as chiropractors, optometrists, pharmacists, and physical therapists, receive doctorates but may not use the title of Doctor, usually abbreviated Dr., before their names or refer to themselves as doctors. However, people who have received a PhD and work outside a medical context, including lawyers, whose degree is a doctorate, are free to use the word or the abbreviation.
By convention, those who have earned an honorary doctorate do not address themselves with the term or its abbreviation, although some have done so, ranging from the lexicographer Samuel Johnson in the eighteenth century to writer and activist Maya Angelou in more recent times.
A surgeon is a doctor who performs operations. Surgeon—and the word for the surgeon’s practice, surgery—derive ultimately from the Greek term kheirourgos, meaning “working by hand”; the intermediate Latin form was chirurgia. The first element in those classical forms is the same as seen in chiropractor, a word coined at the turn of the twentieth century to refer to a specialist in manipulating the skeletal system as a form of physical therapy; the technique is called chiropractic. (The second part of the words derives from the Greek word for practical.) The second element in surgeon and surgery, referring to work, is related to the prefix ergo-, as seen in ergonomics, as well as to the word organ.
Physicians are generally distinct from surgeons in that they specialize in treatment by medication rather than operation, but there is some overlap; the word, which comes from Greek by way of Latin, means “study of nature,” and physical and physics are related.