n. pl. lu·mens or lu·mi·na (-mə-nə)
1. Anatomy The inner open space or cavity of a tubular organ, as of a blood vessel or an intestine.
2. Biology The interior of a membrane-bound compartment or organelle in a cell.
3. Abbr. lm Physics The SI unit of luminous flux, equal to the amount of light per unit time passing through a solid angle of one steradian from a light source of one candela intensity radiating equally in all directions. See Table at measurement.
[Latin lūmen, an opening, light; see leuk- in the Appendix of Indo-European roots.]
lumen·al, lumin·al adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.