Thing is, you usually define all your variables. At least we do in engineering (of physical variety, rather than software).
Mostly because we can't expect everyone reading the calculation to know, and that not everyone uses the same symbols.
Not explaining each variable is bad practice, other than for very simple things. (I do expect everyone and their dog reading a process eng calc to know PV=nRT, at a minimum).
Just like (in my opinion) not defining industry specific abbreviations is also bad practice.
Mathematicians don't do this? Shame on them.
Lol fair point regarding Eng: "Engineering".
But Nah, I think assumed knowledge of PV=nRT is fair in context, since if you don't know what it is, you'll only be reading the conclusion, not getting into the weeds of a calculation document.
I'm not going even going to be explaining if I have a column that's says volumetric flow rate, with V=m/ρ. If I give mass flow rate and density (with units, of course), and use these extremely common symbols, and someone doesn't understand, then they have no real business getting to this level of detail anyway.
I do agree that in most cases not defining your variables is bad practice, but there is some nuance, depending on the intended audience and how common a formula is, and the format of whatever it is you're writing.