Definition: Cyclomethicone is a clear, odorless, silicone-based oil that is incredibly useful and versatile in a number of bath & body recipes. It is a synthetic unmodified silicone that stays on the surface of the skin - the molecules are too big to physically enter past the upper living cells. It doesn't really have any "healing" or "moisturizing" qualities in and of itself - think of it more as a "carrier." Because it evaporates relatively quickly, it is most commonly used to carry fragrance or essential oils in linen sprays, body sprays, and roll-on "dry" perfumes. It also is used in lotions to impart a slick, silky feel - and is widely used in hair treatments.
There are many different varieties and grades of cyclomethicone. Be sure that you get it from a reliable bath and body supplier. Different grades also vary in their flashpoint. The lower the flashpoint, the quicker it will evaporate. But if you are going to be shipping or mailing the product, you need to be sure the flashpoint is at least 170 degrees. Several suppliers carry "Cyclomethicone NF" which is the premium grade.