I read somewhere that the term for a concept that we don't try to define is "primitive". Primitives are supposed to be to definitions what axioms are to theorems. I also read that when one of these deliberately undefined concepts is explained (but not really defined) by a description in plain English, such an explanation is called an "elucidation". My problem is that I tried to find out if this terminology is standard, and I was only able to find a few sources who used the word "primitive" in that sense, and none that used the word "elucidation". (I don't remember where I learned those terms). Now I'm wondering if these are not the standard terms, and if there are other terms that are used more often by mathematicians and philosophers.