lumen

Lumen, luminis is neuter, third declension, from the same root as lux with the stem lumin-. The distinction against lux is worth holding: lux is light itself, daylight, the abstract; lumen is a light, a particular source, a lamp, a torch, the gleam coming through an opening.

Two extensions follow. In verse lumina is a standard word for eyes, the lights of the face, and it is common enough that a poet using it is not being clever. In architecture and later in medicine lumen is the opening itself, which is why a blood vessel has a lumen in English. Church Latin uses it constantly: lumen de lumine in the Creed, lumen Christi at the Easter vigil. English luminous, illuminate and limn all come from it.