IQ (Intelligence Quotient) is a numerical measure of cognitive ability derived from standardised intelligence tests. Originally: $IQ = \dfrac{\text{Mental Age}}{\text{Chronological Age}} \times 100$. Modern IQ tests (e.g. Wechsler, Stanford-Binet) compare scores to age-group norms, with a mean of 100 and SD of 15. IQ measures reasoning, problem-solving, and abstract thinking. The concept was developed by Alfred Binet and Lewis Terman.