1. C₁₀H₁₆ A moderately toxic, flammable, unsaturated hydrocarbon liquid found in essential oils and plant oleoresins; used as an intermediate for camphor, menthol, and terpineol. 2. A class of naturally occurring compounds whose carbon skeletons are composed exclusively of isopentyl (isoprene) C₅ units. Also known as isoprenoid.