A tributary of the river Gelion in Beleriand (S/123), translated “Glittering Torrent” (SI/Brilthor, Ety/MBIRIL). It is a combination of bril “glass, crystal” and a form of [N.] thórod “torrent” (Ety/MBIRIL, THOR). Given the river’s location in Ossiriand, this name might be Nandorin instead.
Conceptual Development: This name appeared in the Silmarillion drafts from the 1930s (SM/135 note #4, LR/263). In The Etymologies, the name Brilthor was designated Ilkorin, with the translation and (essentially) the same derivation as above (Ety/MBIRIL, THOR). Like many of the river names in Ossiriand, Tolkien did not give a new etymology of the name after he abandoned the Ilkorin language.
bril (Ilk. “glass, crystal”) + thôr (“swooping, leaping down”)