From
http://en.wikipedia.org/wiki/Intelligence:
[ The neutrality of this article is disputed. ]
The definition of intelligence has long been a matter of controversy.
Intelligence comes from the Latin verb "intellegere", which means "to understand". By this rationale, intelligence (as understanding) is arguably different from being "smart" (able to adapt to one's environment), or being "clever" (able to creatively adapt). By the Latin definition, intelligence arguably has to do with a deeper understanding of the relationships of all things around us; and with a capability for metaphysical manipulation of such objects once such understanding is mastered.
At least two major "consensus" definitions of intelligence have been proposed. First, from Intelligence: Knowns and Unknowns, a report of a task force convened by the American Psychological Association in 1995:
Individuals differ from one another in their ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought. Although these individual differences can be substantial, they are never entirely consistent: a given person’s intellectual performance will vary on different occasions, in different domains, as judged by different criteria. Concepts of "intelligence" are attempts to clarify and organize this complex set of phenomena. Although considerable clarity has been achieved in some areas, no such conceptualization has yet answered all the important questions and none commands universal assent. Indeed, when two dozen prominent theorists were recently asked to define intelligence, they gave two dozen somewhat different definitions.[1]
A second definition of intelligence comes from "Mainstream Science on Intelligence", which was signed by 52 intelligence researchers in 1994:
a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience. It is not merely book learning, a narrow academic skill, or test-taking smarts. Rather, it reflects a broader and deeper capability for comprehending our surroundings—"catching on", "making sense" of things, or "figuring out" what to do.[2]
References:
[1] APA Task Force Report, "Intelligence: Knowns and Unknowns"
http://www.lrainc.com/swtaboo/taboos/apa_01.html
[2] Mainstream Science on Intelligence reprinted in Gottfredson (1997). Intelligence p. 13
http://www.lrainc.com/swtaboo/taboos/wsj_main.html