Presently, there are many claims of multiple kinds of intelligence, at least 8 different types, including: emotional, verbal, and spatial. However, in the early 20th century , the idea of 'General Intelligence' was devised by psychologist Charles Spearman. In his measurments, 'g' was the unit used to label performance on a variety of intellegence tests.
In his studies, Spearman found that the people who tended to score high on a variety of mental tests seemed to use the same part of their brains, labeled (you guessed it) 'g'. His label sparked the idea of a 'single intelligence', which is characterized as to what enables us to undertake daily mental tasks.
Spearman's study seems a bit silly upon first read, considering the multiple kinds of intelligence we are used to hearing about. However, more recent studies have revealed some truth to his ideas. Upon giving volunteers 'complicated puzzles' to complete, research has found that the lateral prefrontal cortex is the only area of the brain, which has an increase in blood flow. Could this be Spearman's 'g'?
There is still plenty of controversy surrounding Spearman's theory, preventing it from being our conclusive definition of intelligence. For instance, there is an equal amount of research which states that the level of our mental ability is a function of social experiences (such as school) and not due to one's biological make-up.