late 14c., "round, having the form of a circle," from Anglo-French circuler, Old French circuler "circular" (14c., Modern French circulaire), from Latin circularis, from circulus "small ring" (see circle (n.)). Meaning "intended for circulation" is from 1650s. The metaphoric circular firing squad is attested by 1990.
1550s, "circular figure," from circular (adj.). Meaning "a notice circulated, a printed paper intended for general circulation" is from 1818, short for circular letter (1650s), one directed to a certain circle of persons and addressing a common interest.