pyrethrinPronunciation: /pʌɪˈriːθrɪn/Definition of pyrethrin noun Chemistry any of a group of insecticidal compounds present in pyrethrum flowers: pyrethrins can cause allergic reactions [mass noun]:it contains natural pyrethrinOrigin: 1920s: from pyrethrum + -in1pyrethrin in other Oxford dictionaries Definition of pyrethrin in the US English dictionary