Wiktionary
n. (context organic compound English) An aromatic organic compound that contributes to the smell and flavour of grapefruit, and is both a sesquiterpene and a ketone.
Wikipedia
Nootkatone is a natural organic compound and is the most important and expensive aromatic of grapefruit. It is a sesquiterpene and a ketone.
Nootkatone was previously thought to be one of the main chemical components of the smell and flavour of grapefruits. In its solid form it is usually found as crystals. As a liquid, it is viscous and yellow. Nootkatone is typically extracted from grapefruit, but can also be manufactured with genetically modified organisms, or through the chemical or biochemical oxidation of valencene. It is also found in Alaska yellow cedar trees and vetiver grass.