laughing gas

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Nitrous oxide, especially as used as an anesthetic.

Century Dictionary and Cyclopedia

1. n. Nitrous oxid, or monoxid (N₂O): so called because when inhaled it usually produces exhilaration, which is followed by insensibility. It is prepared by carefully heating ammonium nitrate, and is evolved as a colorless gas with a pleasant smell and sweet taste. It may be liquefied by pressure, and in this condition stored for use. It is used as an anesthetic agent in minor surgical operations, particularly in dentistry.

Wiktionary

1. n. A common name for nitrous oxide used as an anaesthetic.

GNU Webster's 1913

1. n. See under Laughing.
2. hyponitrous oxide, or protoxide of nitrogen; -- so called from the exhilaration and laughing which it sometimes produces when inhaled. It is much used as an anæsthetic agent.

WordNet 3.0

1. n. inhalation anesthetic used as an anesthetic in dentistry and surgery