walled plain

noun

a circular or almost circular area on the moon, sometimes with a floor that is depressed, usually partially enclosed by walls that rise to varying heights and that are usually lower than those of a crater.

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British Dictionary definitions for walled plain

walled plain

noun

any of the largest of the lunar craters, having diameters between 50 and 300 kilometres

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012