catalytic converter

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A reaction chamber typically containing a finely divided platinum-iridium catalyst 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.

Wiktionary

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

WordNet 3.0

1. n. 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