General Intelligence - Charles Spearman
General intelligence, which is also known as g factor, refers to the existence of a general intelligence that influences performance on mental abilities. This theory was first described by Charles Spearman in 1904. According to Spearman, the g factor was responsible for overall performance on mental ability tests. Those who hold this view believe that intelligence can be measured and expressed by a single number, such as an IQ score. The idea is that this underlying general intelligence influences performance on all cognitive tasks.
Spearman also proposed that there are specific intelligences that are used within particular areas that are not related to other cognitive tasks.
Text sources: http://psychology.about.com/od/gindex/g/general-intelligence.htm http://tip.duke.edu/node/1280