Heteroatom

In the nomenclature of organic chemistry, a heteroatom is any atom that is not carbon or hydrogen, typically, but not excluded to nitrogen, oxygen, sulfur, phosphorus or boron.

In the description of protein structure, particularly in the now-deprecated Protein Data Bank file format, a heteroatom record (HETATM) describes an atom belonging to a small molecule cofactor rather than to part of a biopolymer chain.