What part of speech is "chiropractic"?

Solution 1:

The OED lists it as both an adjective and a noun. Other dictionaries, such as Wiktionary, list it as only a noun. Merriam-Webster Online, curiously enough, defines a noun, and lists a possible adjectival form as a related word, but doesn't seem to mention it again. Based on this, I would say it's definitely a noun.

But is it also an adjective, as the OED supposes?

The COCA has the various following usages:

objected to chiropractic medicine because
have received chiropractic treatment
chiropractic consultation

You wouldn't say "podiatry medicine", "podiatry treatment", or "podiatry consultation",. Instead, the proper way is to use the adjectival form: "podiatric medicine", "podiatric treatment", "podiatric consultation".

Thus, chiropractic is both a noun and an adjective.

Solution 2:

I agree that "chiropractory" is nonsense. "Chiropractic" is unquestionably a noun, in wide usage.

And consider this: "Chiropractic" as a noun derives from the simple elision of "medicine" from the original phrase "chiropractic medicine," a still-valid phrase in which the original parts of speech are clear. Over time, the abbreviated usage came into being, presumably because it was just easier and faster to say.

Taking it further: "Allopathic medicine" and "osteopathic medicine" are two other main branches of medical practice. These names refer to M.D.'s and D.O.'s. Most people would recognize these as the degrees that physicians usually have. In the U.S., one's primary doctor and most specialists are M.D.'s, but a huge percentage are D.O.'s, especially in the Midwest. They are legally, ethically, and functionally equivalent medical degrees, although the historical philosophic underpinnings of these two disciplines differ. And although the terms "allopathic medicine" and "osteopathic medicine" are not very commonly used, the proper derived form of each is "allopathy" and "osteopathy."

Similarly, "chiropathy" (certainly not "chiropractory") would qualify as the proper noun form of "chiropractic medicine," but for reasons unclear to me, it is not much in use. It has been used, however, and can still be found in recent usage.