Hephaestion, a Greek grammarian, who instructed the emperor Verus in Greek, and accordingly lived about the middle of the second century after Christ. (Capitolin. Verus Imp. 2.) It is commonly supposed that he is the same as the Hephaestion whom Suidas calls an Alexandrian grammarian. This latter Hephaestion wrote versified manuals on grammatical subjects. Suidas, who mentions several works besides, speaks of one entitled metron Pedismoi, which is believed to be the same as the Encheiridion peri metron, which has come down to us under the name of Hephaestion, and is a tolerably complete manual of Greek metres, forming, in fact, the basis of all our knowledge on that subject. This little work is of great value, not only on account of the information it affords us on the subject it treats of, but also on account of the numerous quotations it contains from other writers, especially poets. The first edition of this Enchiridion appeared at Florence, 1526, 8vo., together with the Greek grammar of Theodorus Gaza. It was followed by the editions of Hadr. Turnebus (Paris, 1553, 4to., with some Greek scholia), and of J. Corn. de Pauw. (Traject. ad Rhen. 1726, 4to.) The best edition is that of Th. Gaisford (Oxford, 1810, 8vo., reprinted at Leipzig, 1832, 8vo.) There is an English translation of it with prolegomena and notes by Th. Foster Barham, Cambridge, 1843, 8vo.