santonin Syllabification: (san·to·nin)Pronunciation: /ˈsantn-in/Definition of santonin noun Chemistry a toxic crystalline compound present in santonica and related plants, used as an anthelmintic. Chemical formula: C15H18O3 Origin: mid 19th century: from santonica + -in1santonin in other Oxford dictionaries Definition of santonin in the British & World English dictionary