Carrier oil, also known as base oil or vegetable 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. Carrier oils do not contain a concentrated aroma, unlike essential oils, though some, such as olive, have a mild distinctive smell. Neither do they evaporate like essential oils, which are more volatile. Most carrier oils are derived from vegetable sources, but mineral oil can also be used.

There is a range of different carrier oils, derived from pressed seeds of various plants. Each possesses individual properties and suitability for use in different treatments. For massage, viscosity is a major consideration; for example, grapeseed oil is typically very thin, while olive oil is much thicker. Sunflower, sweet almond and rapeseed oils have viscosities midway between these extremes. Carrier oils can be simply blended to combine their properties of viscosity, acceptability, lubrication, absorption, aroma and so forth.