first quarter n (Astronomy) one of the four principal phases of the moon, occurring between new moon and full moon, when half of the lighted surface is visible from earth. Compare last quarter Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014