Heteroatom

In the nomenclature of organic chemistry, a heteroatom is any atom that is not carbon or hydrogen, typically, but not exclusively, 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.