A name.
Inachos, a king of Argos [Myth, Place], founded a city which he named for the moon, "Io", for that is what Argives call the moon. He also had a daughter Io; Pekos who is also [called] Zeus[1] abducted her and fathered a daughter, Libya, by her. And Io, lamenting her ruin, fled to the Silpion Mountain[2] and there died. Her father and her brothers, when they learned this, built a shrine to her and called the place Iopolis and remained there until the end. And they performed a ritual in her memory, banging on each other's doors every year and saying "io, io!"