In chemistry, residue is the material remaining after distillation, evaporation, or filtration. Residue may also refer to an atom or a group of atoms that forms part of a molecule, such as a methyl group. It may also denote the undesired by-products of a chemical reaction.
Toxic chemical residues are a concern in food safety. For example, the U.S. Food and Drug Administration (FDA) and the Canadian Food Inspection Agency (CFIA) have guidelines for detecting chemical residues that are possibly dangerous to consume.
In biochemistry and molecular biology, a residue refers to a specific monomer within the polymeric chain of a polysaccharide, protein or nucleic acid. One might say, "This protein consists of 118 amino acid residues" or "The histidine residue is considered to be basic due to its imidazole ring." Note that a residue is different from a moiety, which, in the above example would be constituted by the imidazole ring or "the imidazole moiety".
During the process by which monomeric building blocks (e.g., amino acids) are strung together to form a polymeric chain (e.g., a protein), some atoms (typically a water molecule) are discarded from each building block, and only a "residue" of the building block ends up in the finished product. A residue may be one amino acid in a polypeptide.