Dimethicone is a synthetically produced silicone oil that is often used in cosmetics as a skin-smoothing emollient and antifoam agent. (1)

In our Daily Defence SPF 30 Powder, dimethicone does not serve as a standalone oil base, but solely as a coating for the zinc oxide particles. 

1 Bährle-Rapp, M. (2012). Springer Lexikon Kosmetik und Körperpflege. 4th revised and expanded edition. Springer.