The Second Exodus is a term used in official Colonial government statements to describe the escape of the human population from Cylon-occupied New Caprica circa 2 ACH (2001BYR) (TRS: "Collaborators").
The use of "second" implies that the First Exodus was the successful escape of civilian ships under the protection of the battlestar Galactica from the worlds of the Twelve Colonies after their destruction in an overwhelming Cylon attack (TRS: "Miniseries", "The Plan").