In Greek mythology, Perseus (US: /ˈpɜːr.si.əs/, UK: /ˈpɜː.sjuːs/; Greek: Περσεύς, translit. Perseús) is the legendary founder of the Perseid dynasty. He was, alongside Cadmus and Bellerophon, the greatest Greek hero and slayer of monsters before the days of Heracles. He beheaded the Gorgon Medusa for Polydectes and saved Andromeda from the sea monster Cetus. He was the son of Zeus and the mortal Danaë, as well as the half-brother and great-grandfather of Heracles (as they were both children of Zeus, and Heracles's mother was descended from Perseus).
Because of the obscurity of the name "Perseus" and the legendary character of its bearer, most etymologists presume that it might be pre-Greek; however, the name of Perseus's native city was Greek and so were the names of his wife and relatives. There is some idea that it descended into Greek from the Proto-Indo-European language. In that regard Graves proposed the only Greek derivation available: Perseus might be from the Greek verb πέρθειν (pérthein, "to waste, ravage, sack, destroy") some form of which is familiar in Homeric epithets. According to Buck, the -eus suffix is typically used to form an agent noun, in this case from the aorist stem, pers-. Pers-eus therefore is a "sacker [of cities]"; that is, a soldier by occupation, a fitting name for the first Mycenaean warrior.
The further origin of perth- is more obscure. Hofmann lists the possible root as *bher-, from which Latin ferio, "strike". This corresponds to Pokorny's *bher-(3), "scrape, cut". Ordinarily *bh- descends to Greek as ph-. This difficulty can be overcome by presuming a dissimilation from the -th- in pérthein, which the Greeks would have preferred from a putative *phérthein. Graves carries the meaning still further, to the Perse- in Persephone, goddess of death. Ventris & Chadwick speculate about a Mycenaean goddess pe-re-*82 (Linear B: 𐀟𐀩𐁚), attested on tablet PY Tn 316, and tentatively reconstructed as *Preswa.
A Greek folk etymology connected Perseus to the name of the Persian people, whom they called the Pérsai (from Old Persian Pārsa "Persia, a Persian"). However, the native name of the Persians – Pārsa in Persian – has always been pronounced with an -a-. Herodotus recounts this story, devising a foreign son of Andromeda and Perseus, Perses, from whom the Persians took the name. Apparently the Persians also knew that story, as Xerxes tried to use it to suborn the Argives during his invasion of Greece, but ultimately failed to do so.