Bis-Peg-18 Methyl Ether Dimethyl Silane
What-it-does:
emollient,
moisturizer/humectant,
silicone
It's a silicone that is water dispersible (as opposed to most other silicones that are usually oil dispersible). It makes the skin smooth and nice (emollient), moisturizes, helps to reduce tackiness, and also has some foam boosting properties. Often used in light, watery formulas to give them extra silky feel.