In Greek mythology and Athenian topography, Ilissus (Ilisos) is the river of Athens that once flowed outside the walls toward Phaleron, and a river-god associated with its banks. Myth links the stream to Boreas' abduction of Orithyia and to the sacred landscape of Agrai and Cynosarges; Plato's Phaedrus sets Socrates' talk under plane trees by its waters. Distinct from the Kephisos, Athens' other great stream.
Adapted from Wikipedia, "Ilisos" · CC BY-SA 4.0
I.River and river-god
The Ilissus drains the western slopes of Hymettus and in antiquity ran outside Athens' walls — Plato's Critias names it among the borders of the early circuit. Its grassy, plane-shaded banks were remembered as idyllic teaching ground for Socrates. Local tradition treated Ilissus as a demi-god, son of Poseidon and Demeter, worshipped on Ardittos near the Panathenaic Stadium.
Hyginus lists him among the Rivers born to Oceanus and Tethys; Apollodorus and Apollonius place Orithyia's abduction by Boreas beside this stream — the birth setting of the Boread twins Zetes and Calais.
II.Sacred banks
The Ilissus corridor held shrines of Artemis Agrotera at Agrai (linked to Marathon rites and the Lesser Mysteries), Pan, and other Attic cults; the spring Kallirhoe lay nearby. Modern Athens covered most of the bed as a storm drain, though stretches near the Olympieion remain visible and restoration plans have been discussed.
The atlas id ilissus is the mythic Potamos and Athenian sacred river — not a modern hydrology article alone.