Today's word is LUMEN, which is a third-declension neuter noun, gen. luminis.
Latin meaning and usage: The basic meaning of lumen is "light." It can also refer to a source of light, such as a lamp or torch. There are also many metaphorical meanings, such as "daylight" or "life" (in the sense of the "light of life"). In addition, lumen can also refer to the eye in the sense of "the light of the eye," according to the ancient belief that the eye was an emitter of light; you can read about this emission theory of light at Wikipedia.
Latin word formation: The word is from the same root as in the verb luceo. So, luc-men yields the noun lumen. You can find many derivative words such as lumino (and its compounds), adjectives like luminosus, and nouns like luminator and illuminatio, etc.
English cognates and derivatives: We use the word "lumen" in English as a measure of brightness or "luminosity" (adjective: "luminous"). Someone who is illustrious is a "luminary." To shine light on something is to "illuminate" it. Something that glows is "luminescent."
Here are some examples of today's word in Latin sayings and proverbs; for more examples, see the page at the Scala Sapientiae, which also contains notes on some of the proverbs cited below:
A deo lumen.
Lumen Dei, lex diei.
Vos estis lumen mundi.
Surgite; lumen adest.
Lumina inter umbras clariora sunt.
Luminis umbra comes.
Aspiciunt oculi duo lumina clarius uno.
Astra castra, Numen lumen.
Scientia lumen vitae.
Lumen caeleste sequamur.
Deus ipse solem, quasi lumen, accendit.
Ingenii lumen est eloquentia.
Ratio est radius divini luminis.
Imbribus obscuris succedunt lumina solis.
Lumen accipe et imperti.
Dulce lumen, et delectabile est oculis videre solem.
Campus habet lumen, et habet nemus auris acumen.
Si lumen alteri de suo lumine accendit, nihilo minus ipsi lucet.
Qui procul ex oculis, procul est a lumine cordis.
Saepe precare numen, dabiturque in pectore lumen.