Despite the variety of concepts of intelligence, the most influential approach to understanding intelligence (i.e., with the most supporters and the most published research over the longest period of time) is based on psychometric testing, which regards intelligence as cognitive ability.
Intelligence, narrowly defined, can be measured by intelligence tests, also called IQ (intelligence quotient) tests. Such intelligence tests take many forms, but 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 factor". 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 principal factor from cognitive test scores using the method of 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 correlated. One common view is that these abilities are hierarchically arranged with g at the vertex (or top, overlaying all other cognitive abilities). However, this is by no means universally accepted.