N,N-Diethyl-meta-toluamide, also called DEET (/diːt/) or diethyltoluamide, is the most common active ingredient in insect repellents. It is a slightly yellow oil intended to be applied to the skin or to clothing, and provides protection against mosquitoes, ticks, fleas, chiggers, leeches, and many biting insects. 1 History.