Eliza is an early natural language processing program created in the 1960s by Joseph Weizenbaum at the MIT Artificial Intelligence Laboratory. It simulates conversation by using pattern matching and substitution techniques, allowing it to respond to user inputs in a way that mimics human dialogue. The most famous version of Eliza is the "DOCTOR" script, which emulates a psychotherapist. Users would type in their thoughts, and Eliza would reflect them back, often rephrasing questions to encourage further discussion. This simple yet effective approach demonstrated the potential for computers to engage in human-like conversation.