Mutation is a phenomenon that results in changes in the nucleotide sequences of the genes in the genome which eventually leads to change in the genotype and phenotype of an organism.
The rate of mutation varies with the type of organism, for example, organisms with less life span can have a fast rate of mutation.
The rate of mutation can be affected by external (environmental) as well as internal factors (genetic).
Ionising radiations like X-rays can induce mutation. Excess exposure to these rays would increase the rate of mutation.