Somatic mutation is a mutation that occurs in cells after fertilization. It can occur at any time throughout the life cycle of an organism and may be spontaneous, as a result of errors in DNA repair mechanisms, or due to environmental stressors such as chemicals, cigarette smoke, or radiation. 

Somatic mutations only affect cells of bodily (somatic) tissue and end with the death of the organism. They do not affect germ cells and are not passed on to the progeny.