mutation

A heritable change in the nucleotidesequence of genomic DNA (or RNA in RNA viruses), or in the number of genes or chromosomes in a cell, which may occur spontaneously or be brought about by chemical mutagens or by radiation (induced mutation).
Source:
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 161