The frequency (h-1) with which a mutation occurs within a organism or gene. In general, rates of spontaneous mutation vary between one in 10E4 and one in 10E8 per gene per generation, and can be considerably increased by mutagens.
Source:
PAC, 1992, 64, 143. 'Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)' on page 161 (https://doi.org/10.1351/pac199264010143)