Wikipedia defines aromatherapy as,
“ a form of alternative medicine that uses volatile plant materials, known as essential oils, and other aromatic compounds for the purpose of altering a person’s mind, mood, cognitive function or health.”
According to another source I read,
Essential oils are the highly concentrated, volatile, aromatic essences of plants. Scientists agree that essential oils may perform more than one function in living plants. In some cases they seem to be a part of the plant’s immune system. In other cases they may simply be end-products of metabolism. Essential oils contain hundreds of organic constituents, including hormones, vitamins and other natural elements that work on many levels. They are 75 to 100 times more concentrated than the oils in dried herbs.*
Aromatherapy and essential oils have been used throughout history. Again according to Wikipedia, the ancient Greek physician, Dioscorides, wrote about the healing properties of essential oils in the first century, and distilled essential oils have been used as medicines since the invention of distillation in the eleventh century. Eastern cultures used therapeutic oils, and in the early 1900’s European doctors and scientists began to discuss the value of essential oils and introduced the term aromatherapy. And during World War II a French surgeon, Jean Valnet, pioneered the medicinal uses of essential oils, which he used as antiseptics in the treatment of wounded soldiers.