“Reading a road map upside-down and generating synonyms for the word “brilliant” are two very different skills. But each is a measurable indicator of general intelligence, a construct that includes problem solving abilities, spatial manipulation and language acquisition. Scientists generally agree that intelligence can be captured by psychometric tests. But the study of intelligence is dogged by questions of just how much IQ contributes to an individual’s success and well-being, how genes and environment interact to generate smarts and why the average IQ score rose throughout the world during the twentieth century.” (Psychology Today)
If imagination is the true sign of intelligence, then I would say that the reason the average IQ score rose during the twentieth century is quite simply that the 20th century opened up the world knowledge-base to more people in a wide and global context. Knowledge is not intelligence, but it does give the individual a wider range of components to play around with in their imagination.
Intelligence, imo, is the ability to use one’s collected knowledge and combine various strands of that knowledge and apply the combination on reality in some way when confronted with a problem.