It doesn't actually matter, the exact definition. The point is to pick a definition and stick with it, so that we can discover things about it. The psychometric variable underpinning IQ tests is general intelligence, or "g". But it doesn't matter if it's not quite right or exact. I can see where you're going. Intelligence is not well-defined => it can't be said to be heritable or genetic => all groups are just as naturally smart as each other.

The reason you do this is because you see that IQ differences caused by genetics help to "uphold" a "narrative" that "oppresses" groups who have low IQ. Of course, it does do exactly that, because it's true, but people being naturally less intelligent, whilst called "oppression" by Critical Social Justice activists, is totally justified because it is meritocratic. If intelligence were so well-defined and its genetic component so obvious that it was impossible to deny, Critical Social Justice would simply focus on dismantling the moral axiom that "truly meritocratic systems are justified".