A pesticide is a substance or a mixture of substances used for killing  pests: organisms dangerous to cultivated plants or to animals. The term applies to various pesticides such as insecticide, fungicide, herbicide and nematocide. 
The definition of residue of pesticide according to the world health organisation (WHO) is:- Any specified substances in or on food, agricultural commodities or animal feed resulting from the use of a pesticide. The term includes any derivatives of a pesticide, such as conversion products, metabolites, reaction products and impurities considered to be of toxicological significance. The term “pesticide residue” includes residues from unknown or unavoidable sources (e.g. environmental) as well as known uses of the chemical. The definition of a residue for compliance with maximum residue limits (MRLs) is that combination of the pesticide and its metabolites, derivatives and related compounds to which the MRL applies.