is best defined as a sudden variation in an inheritable characteristic, as distinguished from a variation that results from generations of gradual selection.

Mutation

Explanation: Mutation is a change in DNA sequence, it may occur due to error when the DNA is copied or it can arise from various environmental factors such as exposure to chemicals, UV radiation, Highly infectious microbes.

Mutation maybe spontaneous. Acquired mutations occur at some point in a person's life and are present only in certain cells, not in every cell in the body while some may target a tissue, an organ or an entire system.