Carrier oils, also referred to as base oils or vegetable oils, are nut, vegetable or seed oils, many of which have their own therapeutic properties. They are used to dilute the concentrated essential oils and essential oil blends. When diluted in a carrier, a very small amount of essential oil can be massaged or rubbed onto the skin in the correct dosage and can cover quite a large area. Just using the essential oil on its own does not go as far and can possibly be too highly concentrated for your skin.
Another excellent reason to have carrier oil on hand is for safety. If, while using essential oils, some accidentally gets in the eyes or is used in too high of a concentration and makes the applied area uncomfortable, rub pure carrier oil across the closed eyelid or other area until the stinging sensation has stopped. A cotton ball with carrier oil on it is an excellent tool for this purpose.
Cold pressed, unrefined carrier oils are always the best quality, but can have a shorter shelf-life. Please be aware of storage instructions as you make your selections.