terephthalic acid (?t?r?f??æl?k) n (Elements & Compounds) a white crystalline water-insoluble carboxylic acid used in making polyester resins such as Terylene; 1,4-benzenedicarboxylic acid. Formula: C6H4(COOH)2[C20: from terebene + phthalic acid]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ter?eph?thal?ic ac?id (?t?r ?f??æl ?k, ?t?r-) n. a white, crystalline, water-insoluble solid, C8H6O2, the para isomer of phthalic acid: used chiefly in the manufacture of resins and textile fibers. [1855?60; tere(bic acid) a terpene derivative (tereb (inth) + -ic) + phthalic] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.