7-dehydrocholesterol

noun

7-de·​hy·​dro·​cho·​les·​ter·​ol ˈsev-ən-(ˌ)dē-ˌhī-drō-kə-ˈles-tə-ˌrōl  -ˌrȯl 

: a crystalline steroid alcohol C₂₇H₄₃OH that occurs (as in the skin) chiefly in higher animals and humans and that yields vitamin D₃ on irradiation with ultraviolet light