Carrier oil, also known as base oil, is used to dilute essential oils and absolutes before they are applied to the skin in massage and aromatherapy. They are so named because they carry the essential oil onto the skin. Diluting essential oils is a critical safety practice when using essential oils. Essential oils alone are volatile; they begin to dissipate as soon as they are applied. The rate of dispersion will vary based on how light or heavy the carrier oil is.