Definitions

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

Etymologies

mas(s) + con(centration).
(American Heritage® Dictionary of the English Language, Fourth Edition)
From mass + concentration (Wiktionary)

Examples

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  • Romantics have asked and will ask on,
    Why the moon puts a pockety mask on.
    This draws a big boff
    From boffins who scoff
    At those who would dream on a mascon.

    August 19, 2014

  • "And, finally, Hamish LIndsay, author of Tracking Apollo to the Moon notes that imperfect knowledge of the effects of mascons (mass concentrations) may have also contributed. In 2006, Hamish consulted with Jerry Bostick, who served as Flight Dynamics Officer on Gene Kranz's White Team. Bostick tells us, 'It's one of those things that is heard to definitely prove one way or the other, but my opinion is that it was a combination of the tunnel pressure and us not completely understanding - being able to accurately model - the mass concentrations.'
    - 'The First Lunar Landing' transcribed by Eric M. Jones, hq.nasa.gov.

    October 8, 2008