Here is a general definition from Wikipedia:In Greek mythology the Erinyes (pl. of Erinys; literally “the angry ones”) or Eumenides ( pl. of literally “the gracious ones” but also translated as “Kind-hearted Ones” or “Kindly Ones”), or Furies or Dirae in Roman mythology, were female chthonic deities of vengeance, or supernatural personifications of the anger of the dead. A formulaic oath in the Iliad invokes them as “those who beneath the earth punish whosoever has sworn a false oath”. Burkert suggests they are “an embodiment of the act of self-cursing contained in the oath”.
When the Titan Cronus castrated his father Uranus and threw his genitalia into the sea, the Erinyes emerged from the drops of blood, while Aphrodite was born from the crests of seafoam. According to variant accounts, they emerged from an even more primordial level from Nyx, “Night”. Their number is usually left indeterminate. Virgil, probably working from an Alexandrian source, recognized three: Alecto (“unceasing”, who appeared in Virgil’s Aeneid), Megaera (“grudging”), and Tisiphone (“avenging murder”). Dante followed Virgil in depicting the same three-charactered triptych of Erinyes; in Canto IX of the Inferno they confront the poets at the gates of the city of Dis. The heads of the Erinyes were wreathed with serpents (compare Gorgon) and their eyes dripped with blood, rendering their appearance rather horrific. Other depictions show them with the wings of a bat or bird and the body of a dog.
HOWEVER, Barbara G. Walker has something else to say in her The Women’s Encyclopedia of Myths and Secrets.
ERINYES: “‘Avenger,’ title of Mother Demeter as the threefold Furies, who punished all trespassers against matriarchal law. In her fearsome avenging aspect, the Goddess sometimes appeared as the Night-mare, with a black horse head wreathed with snakes.”
She goes on to say:
FURIES: “Also called Erinyes or Eumenides, the Furies personified the vengeful moods of the Triple Goddess Demeter, who was also called Erinys as a punisher of sinners. The three Erinyes were emanations of her. “Whenever their number is mentioned there are three of them…But they can all be mentioned together as a single being, an Erinys. The proper meaning of the word is a ‘spirit of anger and revenge’…Above all they represented the Scolding Mother. Whenever a mother was insulted, or perhaps even murdered, the Erinyes appeared. Like swift bitches they pursued all who had flouted blood-kinship and the deference due to it.
Greeks believed the blood of a slain mother infected her murderer with a dread spiritual poison, miasma, the Mother’s Curse. It drew the implacable Furies to their victim, and also infected any who dared help him. In fear of the Furies’ attention, lest they might have inadvertently assisted a matricide, people called the Furies “Good Ones” (Eumenides), hoping to divert their wrath.
Aeschylus called the Furies “Children of Eternal Night.” Sophocles called them “Daughters of Earth and Shadow.” Their individual names were Tisiphone (Retaliation-Destruction), Megaera (Grudge), and Alecto (The Unnameable). Some said they were born of the blood of the castrated Heavenly Father, Uranus; others said they were older than any god. Their antiquity is demonstrated by the fact that they were invoked against killers of kinfolk in the female line only: a relic of the matriarchal age, when all genealogies were reckoned through women.
Aeschylus’s drama The Eumenides presented the Furies pursuing Orestes for killing his mother, Queen Clytemnestra; but they cared nothing for the murder of the father. He was not a real member of the clan. When Orestes asked them why they didn’t punish Clytemnestra for murdering her husband, they answered, “The man she killed was not of blood congenital.” Orestes inquired (as if he didn’t know), “But am I then involved with my mother by blood bond?” The Furies snapped, “Murderer, yes. How else could she have nursed you beneath her heart? Do you forswear your mother’s intimate blood?” In short, the Furies harked back to a matriarchal clan system like the one in pre-Christian Britain, where “the son loved the father no more than a stranger.” Indeed the name of the archaic Triple Goddess of Ireland, Erin, or Eriu, has been linked with the triple Erinyes.
The Furies were also “fairies,” identified with witches because of their ability to lay curses on any who transgressed their law. Such “fairies” may have been real witches who tried to defend the rights of women against encroachment by Christian laws. Their modus operandi could have been similar to that of the Women’s Devil Bush society in Africa: if a woman complained to this society that her husband abused her, he soon died of a mysterious dose of poison.
Christianity adopted the Furies, incongruously enough, as servants of the patriarchal God. They became part of God’s penal system in hell: dog-faced she-demons known as Furies Who Sow Evil, Accusers or Examiners, and Avengers of Crimes. Their duty, as always, was to punish sinners. As “grotesques” they appeared on the tympanum of Bourges Cathedral, with large pregnant bellies bearing the full moon’s Gorgon face, and pendulous breasts terminating in dogs’ heads. Greek art, however, depicted them as stern-faced but beautiful women, bearing torches and scourges, with serpents wreathed in their hair like the Gorgons.
Although classical tradition understood the Fury as a symbol of the impersonal functioning of justice, yet she came to represent men’s hidden fear of women, an image apparently still viable. Psychiatric Worldview says:
‘To those men who are aware of contemporary changes it becomes abundantly clear that there are a number of openly angry women around….Men trained to recognize and enhance their own anger and aggressiveness in a society where rape and revenge are commonplace view angry women with alarm….Men see women project onto them the full extent of their own potential aggressiveness. The spectre of an angry Fury or Medusa’s head strikes fear in men, which is then often awkwardly handled because men are not supposed to display fear. A woman seeking only reasonable social or vocational equity may be perceived by a man as being out to get the kind of revenge that his pride would require had he experienced the narcissistic and practical wounds that she has sustained.'”