ASS, a common name for varieties of the subgenus Asinus, belonging to the horse tribe, and especially for the domestic ass; it differs from the horse in its smaller size, long ears, the char acter of its tail, fur and markings, and its proverbial dullness and obstinacy. The fifth proposition of Book i. of Euclid is known as the Pons Asinorurn, "bridge of asses."