Intelligence is usually said to involve mental capabilities such as the ability to reason, plan, solve problems, think abstractly, comprehend ideas and language, and learn. Although laymen generally regard the concept of intelligence as having much broader scope, in psychology, the study of intelligence generally regards this personality trait as distinct from creativity, personality, character, or wisdom.

Definitions of intelligence

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:

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.
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. (reprinted in Gottfredson, 1997, p. 13)

Psychometric intelligence

Despite the variety of concepts of intelligence, the most influential approach to understanding intelligence (i.e., the one that has generated the most systematic research) is based on psychometric testing.

Intelligence, narrowly defined, can be measured by intelligence tests, also called IQ tests. Such tests are among the most accurate (reliable and valid) psychological tests. Such intelligence tests take many forms, but g theory proponents argue that the common tests (Stanford-Binet, Raven's Progressive Matrices, Wechsler Adult Intelligence Scale, Wechsler-Bellevue I, and others) all measure the same dominant form of intelligence, g or "general intelligence". The abstraction of g stems from the observation that scores on all forms of cognitive tests correlate positively with one another. g can be derived as the principle factor from cognitive test scores using the method of principal components analysis or factor analysis.

In the psychometric view, the concept of intelligence is most closely identified with g, or Gf ("fluid g"). However, psychometricians can measure a wide range of abilities, which are distinct yet intercorrelated. One common view is that these abilties as hierarchically arranged with g at the vertex (or bottom, underlying all other abilities).