catalytic converter

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

• noun A reaction chamber typically containing a finely divided catalyst (usually platinum and other metals such as rhodium, palladium, and gold) into which exhaust gases from an automotive engine are passed together with excess air so that carbon monoxide and hydrocarbon pollutants are oxidized to carbon dioxide and water, and nitrogen oxides are reduced to nitrogen and oxygen gases.

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

• noun automotive A chamber containing a finely divided platinum catalyst in which carbon monoxide and hydrocarbons from the exhaust are oxidized to carbon dioxide.

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

• noun a converter that uses a platinum-iridium catalyst to oxidize pollutants and carbon monoxide into carbon dioxide and water; an antipollution device on an automotive exhaust system