isocyanic acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A caustic and reactive organic acid, HNCO, mainly occurring and used in its ester or salt forms or as its tautomer, cyanic acid, which it forms in aqueous solution. Isocyanic acid is an intermediate in the production of urethanes and has been detected in some comets.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun inorganic chemistry The unstable compound H-N=C=O that is metameric with cyanic acid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an acid known only in the form of its esters