ketene

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

• noun A pungent, toxic, colorless gas, C₂H₂O, used chiefly as an acetylation agent.

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

• noun organic chemistry any of a class of unsaturated ketones, having a general formula R1R2C=C=O, that react as if they were inner acid anhydrides
• noun organic chemistry the parent compound of this class, CH2=C=O, an unstable, reactive colourless gas