“Numerous definitions of and hypotheses about intelligence have been proposed since before the twentieth century, with no consensus reached by scholars.” - source wikipedia
Despite the lack of an agreed definition, tests of IQ (Intelligence Quotients) are still routinely used to grade students, job-applicants and patients.
Further reading; Intelligence: Knowns and Unknowns Board of Scientific Affairs report, 1995.
Quote from Professor Stanley M. Garn (university of Michigan)
“If the Aborigine drafted an I.Q. test, all of Western civilization would presumably flunk it.” source
Quote from Nobel prizewinner professor Peter Medawar
“I once spoke to a human geneticist who declared that the notion of intelligence was quite meaningless, so I tried calling him unintelligent. He was annoyed, and it did not appease him when I went on to ask how he came to attach such a clear meaning to the notion of lack of intelligence. We never spoke again.”