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 ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.