I. The companion of Aeneas in his flight from Troy, and in his subsequent wanderings, according to the account given by Virgil. He is always termed fidus Achates (the faithful Achates), whence the phrase has passed into a proverb, applied to any faithful confidant in a subordinate position.

II. In ancient geography, a river in the south of Sicily, between Camarina and Gela, now called Dirillo. According to Pliny, it was the place where the first agate was found; hence the derivation of the word agate.