Heliopolis (i. e. City of the Sun), in Egyptian On, one of the oldest and most sacred cities of Egypt; was situated about 10 m. N. of Cairo, on the eastmost branch of the Nile; it was the centre of Egyptian learning; Solon and Plato are said to have studied there, and Potiphar was one of its chief priests; the famous obelisk Pharaoh's Needle stands near; and Cleopatra's Needle, now on the Thames Embankment, was originally of this city. Also the name of Baalbec.