1 Clear and deep understanding or perception.
2 The process by which the meaning or significance of a pattern or the solution of a problem suddenly becomes clear, often accompanied by an aha experience. A famous and dramatic example is that of the ancient Greek mathematician and physicist Archimedes of Syracuse (?287–212bc), who was asked by King Hiero II (308–216bc) to determine whether his gold crown was alloyed with silver. Archimedes could not at first think how to perform such a test, but when getting into a bath that was full to its brim, he had an aha experience, realizing suddenly that a body heavier than water must displace its own volume of water when immersed, and that because silver is lighter than gold, a pound weight of gold/silver alloy must displace more water than a pound weight of gold, enabling the crown to be tested against a piece of pure gold of equal weight, whereupon (according to Vitruvius Pollio) he jumped out of the bath crying ‘Eureka! Eureka!’ (I've found it! I've found it!) and ran home naked to perform the experiment.
3 The capacity to understand oneself, especially the abnormal or pathological nature of aspects of one's behaviour or mental experience that result from a mental disorder; often used to distinguish neurosis, in which insight is typically present, from psychosis (1), in which it is typically absent. In psychoanalysis, conscious understanding of unconscious reasons for maladaptive behaviour is believed to be curative in itself. See also Anton's syndrome, interpretation (2), problem-solving stages.