from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A mare on the moon having rock of greater density than that of the surrounding area, thus exerting a slightly higher gravitational force.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A Lunar mare that has a greater density of rock than the surrounding area
mas(s) + con(centration).(American Heritage® Dictionary of the English Language, Fourth Edition)
From mass + concentration (Wiktionary)