Aromatic volatile material obtained from botanicals through the process of distillation or cold-pressing/expression. 

Steam distillation: Usually indirect steam which is produced in a boiler. The steam goes through the plant material which can be resting on a grid or trays. The steam condenses and separates when cool into the water and the essential oil that floats on the top. 

Expression: Is reserved for citrus fruits such as lime, bergamot and orange. These are called cold pressed essential oils.