This document defines intelligence and discusses theories of intelligence. It provides numerous definitions of intelligence from various researchers. It also outlines several theories of intelligence, including Thorndike's multi-factored approach, Spearman's two-factor theory, Thurstone's group factors, Vernon's hierarchical model, Guilford's structure of intellect model, Cattell's fluid and crystallized intelligence, Gardner's theory of multiple intelligences, Sternberg's triarchic theory, Carroll's three-stratum theory, Sternberg's theory of mental self-government, and Das, Naglieri and Kirby's PASS theory. The document discusses intelligence tests and the differences between general mental ability tests, specific mental ability tests, aptitude tests