Wiktionary
acyl chloride
n. (context organic chemistry English) Any organic compound containing an acyl functional group directly attached to a chlorine atom.
Wikipedia
Acyl chloride
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group - CO Cl. Their formula is usually written RCOCl, where R is a side chain. They are reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CHCOCl. Acyl chlorides are the most important subset of acyl halides.