the instant, approximately one week after a full moon, when half of the moon's disk is illuminated by the sun.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for last quarter
one of the four principal phases of the moon, occurring between full moon and new moon, when half the lighted surface is visibleCompare first quarter
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