ELIZA is a natural language conversation program described by Joseph Weizenbaum in January 1966 [1].
It features the dialog between a human user and a computer program representing a mock Rogerian psychotherapist.
The original program was implemented on the IBM 7094 of the Project MAC time-sharing system at MIT and was written in MAD-SLIP.

This is how Joseph Weizenbaum discussed his choice for a conversation model as it would be found in psychotherapist's session: