When geneticists speak of mutation they are referring to?

In genetics, mutation refers to a change in the sequence of genes. This can mean a change in the genes in an organism's DNA or in the DNA/RNA of a virus.

There are many different types of mutations, which will have differing effects depending on how many bases/sections have been changed, where the sections are located and how different the resultant protein(s) will be.

For example; a change in one base, that does not change the amino acid that will be coded for is called a silent mutation. This will have no affect on the resulting protein or the organism.