INCI name
Dimethicone
Also listed as: Polydimethylsiloxane, a silicone
Dimethicone is a silicone oil. It spreads easily, leaves a smooth film and reduces water loss from the skin. It is common in primers, foundations, hair conditioners and moisturisers. Related silicones include cyclopentasiloxane, dimethiconol and many PEG-modified dimethicones, each with its own INCI name.
Dimethicone is not the same as "plastic" in the household sense, though it is a synthetic polymer. It is not water soluble, which is why a silicone-heavy hair product may need a thorough wash. Skin feel is usually velvety rather than greasy.