Dimethicone is a type of silicone (technically known as a non-volatile/linear silicone). It is one of the most frequently used emollient (emollient) ingredients in moisturizers, known for its gentleness and effectiveness.
Dimethicone is recognized as a "skin protectant" by the US FDA when used in over-the-counter products at concentrations of 1-30%. It forms an air-permeable barrier that works to prevent moisture loss so the skin better retains hydration while still allowing natural sweat to evaporate. It gives a wonderfully silky (but not greasy) feel, making it particularly popular in oil-free moisturisers.
Dimethicone can also be used with other ingredients such as aluminum oxide to coat zinc oxide or titanium dioxide in sunscreen formulation to help the active ingredients distribute evenly on the skin, providing uniform protection.
Dimethicone is considered a synthetic ingredient, even though it is derived from natural silicon. It has a long history of being recognized as safe in cosmetic products. https://www.paulaschoice.com/ingredient-dictionary/ingredient-dimethicone.html?q=Dimethicone&fdid=ingredients