Mutation is a process by which the genes of a lifeform are changed in some way by either internal or external stimuli. The lifeforms affected by this process are known as mutants. Mutation is a primary factor in evolution, as those mutations that increase an organism's overall likelihood of survival are usually incorporated into the genome of the lifeform's offspring. Other mutations may be trivial, or detract from an organism's viability for reproduction or continued existence.
While observing Klingons in the 23rd century that lacked ridges on their foreheads, Dr. Julian Bashir asked Worf if they suffered from a genetic mutation. (DS9: "Trials and Tribble-ations")
Jack and other genetically engineered Humans sometimes referred to themselves as "mutants". (DS9: "Statistical Probabilities", "Chrysalis")
It was a mutation in Odo's body that led to him developing Section 31's morphogenic virus. (DS9: "The Dogs of War")
In 2374, the Srivani triggered mutations in the crew of the USS Voyager using genetic tags. (VOY: "Scientific Method")