Yours, Etc.: Origins and Uses of 8 Sign-Offs
c.1600, "act or fact of coming together again," from re- "back, again" + union; or from French réunion (1540s). Meaning "meeting of persons of previous connection" is from 1820.
The island of Reunion, formerly known as Bourbon, was renamed during the French Revolution (1793) in commemoration of the 1792 union of revolutionaries from Marseille with the National Guard in Paris, renamed back to Bourbon after 1815, then back to the Revolutionary name after 1848.
sixth king of Portugal (1279-1325), who strengthened the kingdom by improving the economy and reducing the power of the nobility and the church