1Frequently with distinguishing Greek letter. Each of a number of fragrant unsaturated isomeric ketones which are products of the breakdown of carotenoids and occur naturally in some essential oils; specifically (more fully "β-ionone") a compound that arises as a product of ψ-ionone in the synthesis of vitamin A.
2In full "ψ-ionone". An acyclic compound isomeric with α- and β-ionone which can be cyclized to form those compounds and is used in the synthesis of vitamin A.
Late 19th century; earliest use found in Bulletin Pharmacy. From German Ionon from ancient Greek ἴον violet (probably, like classical Latin viola, a loan from a language of the Mediterranean) + German -on.