Silicone is any of many polymers made up of alternating oxygen and silicon atoms combined with other organic groups.
Silicone can be an oil, grease, gel or plastic, but all forms are resistant to heat, water and will not conduct electricity. It is used for a huge number of uses in industry (lubricants, paints, adhesives, coolants, etc.)
Silicone is used in medicine to replace body parts, and its use in breast implants has come under fire because there is evidence that these appliances may leak, and further evidence that leaked silicone may cause autoimmune disorders.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.