Carrier oils, also referred to as base oils or vegetable oils, are used to dilute essential oils before they are applied to the skin.
They “carry” the essential oil onto the skin. Different carrier oils offer different properties and the choice of carrier oil can depend on the therapeutic benefit being sought.

Carrier oils are generally cold-pressed vegetable oils derived from the fatty portions of the plant. Unlike essential oils that evaporate and have a concentrated aroma, carrier oils do not evaporate or impart their aroma as strongly as essential oils.

Examples of carrier oils are sweet almond, apricot kernel, grapeseed, avocado, peanut, olive, pecan, macadamia nut, sesame, evening primrose, walnut and wheat germ.
Most oils bought in the grocery store are not cold-pressed. Instead, the oils are heated and therefore have less therapeutic benefit.
Mineral oil is not used in aromatherapy because mineral oil is not a natural product.
It is also said that mineral oil can prevent essential oil absorption into the skin.

Essential oils do not go rancid. Carrier oils, however, can go rancid.
Carrier oils that you purchase should be natural and unadulterated.
Exceptions include buying carrier oils that have natural vitamin E added. Vitamin E acts as a natural preservative thus extending the shelf life of the carrier oil.