Nucleotide base changes in the genome from its parental copy which for eukaryotes or bacteria is DNA

The technical definition is that for a given homologous position within an alignment, i.e. the inherited position from a parental copy, a base change occurs involving A, G, C or T. Mutations normally occur through errors in the copy proceedure involving the polymerase enzyme acting on the central unit of inheritance that which in the vast majority of cases is DNA, except in viruses where large numbers use RNA.