The Taeglin was a river in Beleriand, a tributary of Sirion. It rose in the Ered Wethrin and flowed southeast through Brethil to join Sirion on the borders of Doriath. It had two minor tributaries: Celebros and Malduin.
Other versions of the legendarium
In earlier writings, the river was called Teiglin. As revealed in the The History of Middle-earth series, the rivername should actually have been spelt Taeglin ("Boundary River"). This was a relatively late change which was not adopted into the published The Silmarillion, which uses the old name.