Lake Biwa is an ancient lake, over 4 million years old. It is estimated to be the 13th oldest lake in the world. Because of its proximity to the ancient capital, references to Lake Biwa appear frequently in Japanese literature, particularly in poetry and in historical accounts of battles.
The name Biwako was established in the Edo period. There are various theories about the origin of the name Biwako, but it is generally believed to be so named because of the resemblance of its shape to that of a stringed instrument called the biwa. Kōsō, a learned monk of Enryaku-ji in the 14th century, gave a clue to the origin of the name Biwako in his writing: “The lake is the Pure land of the goddess Benzaiten because she lives on Chikubu Island and the shape of the lake is similar to that of the biwa, her favorite instrument.”
The lake was formerly known as the Awaumi (淡海, Freshwater Sea) or the Chikatsu Awaumi (近淡海, Freshwater Sea Near [the Capital]). Later the pronunciation Awaumi changed to the modern Ōmi as in the name of Ōmi Province. The lake is also called Nio no Umi (鳰の海, “Little Grebe Lake”) in literature.