Tetrahydrocannabinol (THC) is the principal psychoactive constituent of cannabis and one of at least 113 total cannabinoids identified on the plant. Although the chemical formula for THC (C₂₁H₃₀O₂) describes multiple isomers, the term THC usually refers to the Delta-9-THC isomer with chemical name (−)-trans-Δ⁹-tetrahydrocannabinol. Like most pharmacologically active secondary metabolites of plants, THC is a lipid found in cannabis, assumed to be involved in the plant's evolutionary adaptation, putatively against insect predation, ultraviolet light, and environmental stress.