nitrous acid

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

• noun A weak inorganic acid, HNO₂, existing only in solution or in the form of its salts.

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

• noun inorganic chemistry A weak acid, HNO2, known only in solution, or as its salts and esters, the nitrites; it has a linear structure, H-O-N=O

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

• noun an unstable inorganic acid known only in solution and as nitrite salts