A lighthouse; so called from the lighthouse built by Sostratus Cnidius in the island of Pharos, near the port of Alexandria, in Egypt. It was 450 feet high, and could be seen at the distance of 100 miles. Part was blown down in 793. This Pharos was one of the Seven Wonders of the World.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894