Horsehead nebula (?h??s?h?d) n (Astronomy) astronomy a dark nebula lying in the constellation of Orion and resembling the head of a horseCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014