Troo, here's my source for the etymology of "doctor": The Oxford English Dictionary. And I quote:Note that "OF" means "Old French." I'd love to know what source you've used that has more credibility than the OED (honestly, I would, because I'd love to have a more authoritative source that is also cheaper than the OED). I'd link you to the OED's page on "doctor," but it's a $500+/yr subscription fee I think. I get it for free through my university (until June 3, when I'm no longer a student :( :( :( )

But it doesn't really matter where it comes from. Even if I were to accept your thesis that "doctor" comes from Swahili, the point I was trying to make is that, despite the fact that the word comes directly from a still-in-use non-English language, we still consider "doctor" to be English because, well, people use it when speaking English.

@Barone1551 Could you explain why you think I'm wrong? I'll state my opinion clearly and then you can poke holes in it:
"When native speakers of language X use word Y, word Y is a part of language X."

Finally, after poking holes in my definition, could you provide a better working definition/method for determining whether word Y is part of language X?

I'm not interested in being right in the short term; I'm interested in being right in the long term. Thus, I welcome corrections with open arms. I'm seeking truth, not victory. I'll never meet a single one of you in real life.