lumin

The Latin noun lūmen meant a source of light — a lamp, a torch, the gleam in someone's eye, the daylight pouring through a window. Its sibling verb lūmināre meant "to light up." Notice the emphasis: where the cousin root lux/luc points at the abstract clarity of light (lucid, translucent), lūmen points at the warm, visible glow — the thing that actually shines in the dark.

That single image of "a glowing source" radiates out into a tidy family once you add prefixes and suffixes:

- il- (a form of in-, here "into / upon") + lūmin + -ate → illuminate: to throw light onto something. First it meant literally lighting a room. Then medieval monks who painted gold and bright colors onto manuscript pages were said to illuminate them — hence an "illuminated manuscript." Finally the metaphor went mental: to illuminate a problem is to throw light onto it, to clarify it.
- illuminate + -ion → illumination: the act or result of lighting up — both the literal glow on a building and the figurative flash of insight.
- lūmin + -ous → luminous: "full of light," glowing. A luminous dial glows in the dark; a luminous explanation glows with clarity.
- luminous + -ity → luminosity: the measurable amount of glow. Astronomers use it for how much light a star pours out.
- lūmin + -ary → luminary: literally "a light-giving body" (the sun and moon were the two great luminaries). The metaphor then jumped to people: a luminary is someone whose brilliance lights up a field — a towering expert, a star.
- il- + lūmin + -ati (Latin plural "the ones who have been...") → illuminati: "those who have been enlightened." A real 18th-century society took the name; pop culture turned it into the shadowy "Illuminati."
- lūmin + fer (carry) + -ous → luminiferous: "light-bearing," famous from the 19th-century "luminiferous aether," the imagined medium that carried light.

The pattern is clean: lumin is always the glow, and the prefix or suffix tells you what is being done with it — thrown onto something (illuminate), possessed by something (luminous), measured (luminosity), or embodied by someone who shines (luminary). The same idea exists in Greek as photo (photograph, photon) and phos — a synonym from a different language, not a cognate.