Epic variant of μάρτυς , often referring to what the gods witness. The headword μάρτυροι occurs five times in the Iliad, e.g. 1.338, and twice in the Odyssey; see also Hesiod, Shield 20.
The definition given already occurs in Herodian, De Prosodia Catholica p. 47 Lentz, περὶ ὀνομάτων p. 615 Lentz, περὶ κλίσεως ὀνομάτων p. 747 Lentz. Herodian argues based on the singular nominative ἐπιμάρτυρος of Iliad 7.76.