This is chapter 2 in my chemistry book.
btw, I cannot get that spreadsheet up here. If I ever find out how to turn it into a picture, I'll do it, but for now, you're stuck having me send it through the email process.
So I just read this tiny first section in this chapter about actual essential oils. There are also oily substances called absolutes, and then there are the citrus oils which are technically not essential oils. These two other kinds of oils are often used in the same way as essential oils, but this one section of the chapter is specifically about essential oils. It was interesting to note a little while ago, though I forgot to mention it, that not every single plant on the earth has essential oils. They do have other types of fluids that perform the same tasks, but they're not always actual essential oils.
"An essential oil is the volatile lipid (oil) soluble portion of the fluids of a plant containing odiferous compounds produced by steam distillation of plant matter." page 51 Essential oils are sometimes called volatile oils, etheric oils, ethereal oils, aromatic oils, and sometimes essences, though that term is usually used for other types of plant substances.
Sometimes the heat of distillation produces "artifacts" which are not in the original plant, but can be therapeutic. Too much high heat will produce too many artifacts, which uses up the healing properties of the original compounds in the oil. Sometimes people short-cut with harsh solvents to remove the oils, which contaminates them and makes them actually harmful to humans.
Proper distillation, with the correct heat and pressure, is the closest we can get to extracting oils exactly as they are found in nature, but it's not completely exact. A few of the artifacts produced with lower heat actually add to the therapeutic properties of the oils.