The American Heritage® Dictionary of the English Language, 4th Edition
n. Any of a class of organic or inorganic chemical compounds of silicon, oxygen, and usually carbon and hydrogen, based on the structural unit R2SiO, where R is an alkyl group, usually methyl.
n. any of a class of compound having a short repeating unit of silicon and oxygen atoms (either in a chain or a ring) with organic side chains
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. any of a large class of compounds that have alternate silicon and oxygen atoms
Word Usage
"Cyclomethicone and ingredients ending in "siloxane": Used in cosmetics to soften, smooth and moisten, these compounds can irritate the skin, eyes and lungs."