Niphredil is probably a Sindarin elvish word
since, in the Fellowship,
Haldir named it at the same time as he used the Sindarin word, elanor, for
the flower growing with it, and both words are italicized. In the Silmarillion,
niphred means "pallor", listed under nim.
Niphredil grew in Lothlorien on Cerin Amroth.
Among the elanor, "growing on slender stalks, were other flowers, white and palest green..." Together the elanor and niphredil flowers within seemed to glimmer "as a mist amid the rich hue of the grass".
Haldir calls it "pale niphredil" and says that both it and elanor are winter flowers, but they bloom all year around in Lothlorien.
Niphredil is told of in the Silmarillion as well:
"Though Middle-earth lay for the most part in the Sleep of Yavanna, in Beleriand under the powers of Melian there was life and joy, and the bright stars shone as silver fires; and there in the forest of Neldoreth Luthien was born, and the white flowers of niphredil came forth to greet her as stars from the earth."
-Chapter 10 "Of the Sindar", Silmarillion
Silmarillion: Ch. 10 "Of the Sindar"