Howard Gardner, a theorist of intelligence, has been working from the 1960's to the present to redefine what constitutes intelligence. He has done a great deal to demolish some older myths of psychology, specifically that intelligence can all be measured by the magical IQ, which measures people's ability to make verbal abstractions and mathematical abstractions.
However, when dealing with the question of what exactly defines intelligence, Gardner's answer seems to be somewhat lacking in theoretical depth. From "Reframing Intelligence" (2000), (my paraphrase):
"Intelligence is what enables people to either solve problems, or create products that are useful within a particular cultural context"
Which is in no way a bad pragmatic definition. If I have a big mass of tape drives that I need put into a box, I would consider someone intelligent if they could get them in, whether it was through mathematical intelligence to best fit them in, physical intelligence to manipulate them in, or interpersonal intelligence to talk someone into doing the job for them. As long as the problem is solved, I am pragmatically pretty satisfied that that person is intelligent.
However, on a slightly more theoretical level, I think this describes ability, which is not the same thing as intelligence. There are many things that give people the ability to solve problems, (such as luck or money) that don't count as intelligence. Gardner defines intelligence as a totally external process, only be whether or not the intelligence is able to fix a specific problem, or set of problems. Is a person who flips a coin in order to make a tough decision equally intelligent as the person who spends hours thinking and researching? Doesn't inner comprehension, even when it is not coupled with any kind of external pragmatics, count as intelligence?
The second half of the definition runs into even rockier ground. There are all sorts of people who are able to create products. No doubt, many of these people are intelligent according to a number of definitions. But trying to tie in the products they create to their intelligence is somewhat tenuous. I have looked around the internet, and I know that no matter how ludicrous of a product, even if it is Sonic the Hedgehog pornography, there is a cultural or subcultural group that values it. There are a great many less ludicrous examples of this. For example, did Van Gogh possess artistic intelligence, if no one in his lifetime admitted he made culturally valuable products? Or, to switch it around, were the people who wrote Nazi propaganda creating culturally useful artifacts? At the time, they would have been recognized as doing such, although now we would say definitely not. Thus, the job of fixing what culture has the right to judge a particular product seems to limit the usefullness of this definition.
I think that Howard Gardner was probably aware of the limits of his definition, and was trying to develop a pragmatic yardstick to judge people's general competency, rather than an attempt to define what intelligence truly means. It would be interesting to read a more indepth discussion by Dr. Gardner of what intelligence is.