It ‘s never been completely clear to me what IQ is supposed to measure. So far as I can tell, it’s the ability to do certain puzzles designed to quantify your IQ.
I think that this is probably close enough - " IQ is what IQ tests measure ".
My recollection (from my undergraduate days of nearly 50 years ago) is that IQ tests had a couple of sources.
In France Alfred Binet was concerned with the comparative intellectual development of children - so he established benchmarks for different ages during the educational progression. A 10 year old child who typically demonstrates the performance of the generality of 10 year old children has an IQ around 100.
In the USA there was a different emphasis. They were looking at criminals and people in psychiatric institutions and attempting to see whether intelligence was a factor in incarceration.
Intelligence is clearly a much more complex phenomenon that IQ tests imply. However, if you have an IQ of 132 or more, as measured on the Stanford-Binet test, you can join Mensa. Its objectives are:
"to identify and to foster human intelligence for the benefit of humanity;
to encourage research into the nature, characteristics, and uses of intelligence;
and to provide a stimulating intellectual and social environment for its members"
So IQ measures the likelihood of becoming a member of Mensa!