acyl halide n (Elements & Compounds) any derivative of carboxylic acid in which the hydroxyl group has been replaced by a halogen atom. Also called: acid halide Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014