Q: What is intelligence?
A: Thing that is measured with intelligence tests.
Q: What are intelligence tests? A: Things that measure intelligence.
Q: What are intelligence tests? A: Things that measure intelligence.
This being the internet, the serious answer is a mouseclick away: http://en.wikipedia.org/wiki/G_factor_%28psychometrics%29
What I am getting at in a roundabout way is that I cannot find any credible definition of intelligence that is not dependent on the these tests. If intelligence were able to measured some other way that corroborated these tests, then I will be more likely to believe them.