"Mutation"¹ is almost as difficult. First, we want to distinguish between a mutation and DNA damage. DNA damage occurs when various enzymes make a mistake and damage the nucleotides in a DNA molecule. Damage also occurs when outside forces such as X-rays or chemical mutagens attack DNA. Examples are thymidine dimers or cleavage of a base from a nucleotide. DNA can also be broken into two or more pieces.
This damage is never copied and passed on to the next generation. Either it is fixed in some way or it is lethal. When the damage is fixed it may end up being identical to the original DNA molecular or it may be altered in some way that is passed on. Thus, mutation is (semi-)permanent change that is heritable.
The genetic material is DNA in most cases but RNA genomes (viruses) can also be mutated. There are many different kinds of "genomes" that have to be covered in our definition. This include virus genomes, mitochondrial genomes, chloroplast genomes, plasmids, and mobile genetic elements (mostly transposons).
Any alteration in the sequence of a genome counts as a mutation, not just those that occur in a gene (whatever that is!). This is important because some of the traditional definitions of mutation are restricted to genes.
Here's a good definition from the Wikipedia site ...
In genetics, a mutation is a change of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal genetic element. Mutations result from unrepaired damage to DNA or to RNA genomes (typically caused by radiation or chemical mutagens), from errors in the process of replication, or from the insertion or deletion of segments of DNA by mobile genetic elements. Mutations may or may not produce discernable changes in the observable characteristics (phenotype) of an organism.The Understanding Evolution at UC Berkeley defines mutation as ...
A mutation is a change in DNA, the hereditary material of life. An organism's DNA affects how it looks, how it behaves, and its physiology. So a change in an organism's DNA can cause changes in all aspects of its life.This isn't good because it doesn't cover RNA genomes and it doesn't distinguish between DNA damage and fixed, heritable, change.
The Genetic Science Learning Center at the University of Utah offer this definition.
A mutation is a permanent change in the DNA sequence of a gene. Mutations in a gene's DNA sequence can alter the amino acid sequence of the protein encoded by the gene.That's no good because it restricts mutations to protein encoding genes.
A quick Google search will reveal many other definitions but none as as good as the Wikipedia entry.
As usual, the standard dictionary definitions are not helpful. They are usually quite poor at defining biological terms. Merriam-Webster defines mutation as ...
a relatively permanent change in hereditary material involving either a physical change in chromosome relations or a biochemical change in the codons that make up genesThis is actually better than some of the definitions from scientists. It's main deficiency is that it restricts mutations to genes.
1. The word comes from the Latin mutare, to change.