An intelligence quotient or IQ is a score derived from a set of standardized tests developed to measure a person's cognitive abilities ("intelligence") in relation to their age group.
An IQ test does not measure intelligence the way a ruler measures height (absolutely), but rather the way a race measures speed (relatively).
Modern IQ tests produce scores for different areas (e.g., language fluency, three-dimensional thinking, etc.), with the summary score calculated from subtest scores.
The average score, according to the bell curve, is 100.