Is asking your PhD supervisor a basic question considered inappropriate?

If your colleagues in the department—presumably also PhD students—don't understand the concept, either, it doesn't strike me that the question is truly "basic." That said, if your advisor is aware of your background, then he should know that there will be some things that might not be "obvious" to you.

Now, in this case, you have already "done your homework": you've searched for an answer on your own but couldn't find one. Mention this to your advisor when you ask the question. A good advisor will recognize that you have done what you're supposed to and provide some guidance to help you. He may not provide you all the answers but should at least be able to point you in the right direction.

Consider the risk-reward tradeoff here.

If you keep quiet and use the wrong interpretation of this "basic" concept, you risk spending the rest of your research time producing nonsense, because you did the equivalent of assuming 2 + 3 = 23.

If you ask your supervisor, you risk looking a bit stupid for five minutes.

The choice seems like a no-brainer to me!

I was on both sides of the fence.

As a PhD student, I asked my supervisor several times about concepts which were maybe obvious for people from that specific field, but not for a newcomer.

Like you, I did my homework and explained my supervisor (a great supervisor, by the way) what I needed, what I searched and why what I found was not helpful. Usually within a few moments I was on the right track.

This was mutually beneficial: he also got to understand what students (MSc level in his field he was a teacher for, outside of the PhD program) may find difficult (and changed some of his notes accordingly), and to realize that advances in other fields may help him as well.

He was welcoming such questions.

As someone who tutored two PhD students (cross disciplinary), I always asked them at some point "do you understand?". I specifically told them that I am asking this question so that they do not feel bad about not knowing (especially when I was sensing a disconnect between the eyes and the brain - for whatever reason).

I was also asking them whether they knew what X was, when I was about to discuss X. I did not want to start before we were on the same page.

To summarize: do ask, show that you did search for an answer, and if your thesis director is a good one he could eventually even encourage this.