What is "anduin"

In J. R. R. Tolkien's fictional Middle-earth, Anduin is the Sindarin name for the Great River of Wilderland, the longest river in the Third Age (the original Sindarin name means Long River). The ancestors of the Rohirrim called it Langflood. It flowed from its source in the Grey and Misty Mountains to the Mouths of Anduin (Ethir Anduin) in the Great Sea (Belegaer). In her Atlas of Middle-earth, Karen Wynn Fonstad estimates a total length of 1,388 miles (2,233 km).