(Link) These are organic compounds, the major building blocks within nearly every living creature. For instance, steroids are derivatives of the triterpene squalene. (Quote from Wikipedia .) They are derived from units of isoprene (that’s the picture up there, with a formula of C5H8) called the isoprene rule or C5 rule. Terpenes are created out of multiples of those isoprene units. When they are modified chemically through oxidation or rearrangement of the carbon skeleton, they become terpenoids or isoprenoids (terpenoids are not the same as terpenes as they have been modified in some way, but some people group them together.) These can be cyclical or linear. Terpenes are everywhere in our products, especially in oils and essential oils.