nitrogen dioxide

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

• noun A deadly poisonous reddish brown gas, NO₂, denser than air, often found in smog and automobile exhaust fumes and synthesized for use as a nitrating agent, a catalyst, and an oxidizing agent.

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

• noun inorganic chemistry the binary compound NO2, a brownish gas that is a poisonous air pollutant; it exists in equilibrium with dinitrogen tetroxide.

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

• noun a highly poisonous brown gas (NO2)