I've been looking in the go-ethereum code (where it appears 24(!) times, mostly in comments) and around, I can't find any reference for Ether having any signification rather than a naming convention.
While it is mentioned in the docs (and in a nomenclature file) that 1 wei = 10^-18 Ether, it seems that the opposite is the real definition, 1 Ether being defined as 10^18 wei since calculations are always made in wei. And indeed while working with Ethereum all units are defined in wei unless some explicit number casting is done.
So, is Ether just a naming convention, easy to remember and convenient considering the market value, or has it a concrete meaning?