Mutation frequency refers to the number or "frequency" of cells with a specific mutation in a given population during a point in time. The phrase is often used in relation to drug resistance

Drug resistance

Drug resistance is the reduction in effectiveness of a drug such as an antimicrobial or an antineoplastic in curing a disease or condition. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used...

 in bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l or viral

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 cultures.

Mutation frequency is related to the concept of mutation rate

Mutation rate

In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation...

.

According to "Genetics: Analysis & Principles" by Robert J. Brooker, mutation frequency is the number of mutant genes divided by the total number of genes within a population.