Mutation was a biological change outside that of normal evolution, be it forced or effected by outside sources to bring a change which would not normally have occurred in the same time frame or along a particular path of evolution. Something was said to be mutagenic if it caused mutation.
Some reasons for mutation were similar to that of evolution - the adaption to a situation, area, or time; however it was a forced and 'unnatural' change, outside of what of would have occurred during 'normal' evolution. Other mutations were caused by accident or by deliberate design of another individual or species.
Mutations could be caused by deliberate subjection to a mutagenic process, (TV: The Daleks, Genesis of the Daleks) by exposure to radiation, (TV: The Ark, PROSE: The Sons of the Crab), by exposure to DNA or toxins from another species, (PROSE: The Story of Martha - The Weeping, TV: Cold Blood) or by other means.
The Daleks were created by Davros mutating the Kaled race by subjecting them (or their cells) to a mutagenic process, forcing them to evolve from the effect of radiation. (TV: The Daleks, Genesis of the Daleks) The use of nuclear and chemical-biological weapons during the war also mutated animal life; many mutants lived in the Lake of Mutations. (TV: The Daleks)
The spiders of Metebelis III mutated, becoming highly intelligent and having great psycho-kinetic / telepathic powers as a result of mutation from the Metebelis or 'blue' crystal. (TV: Planet of the Spiders)
The Primords came into existence during an attempt to penetrate the Earth's crust, when humans came into physical contact with Stahlman's ooze. This mutated them into the animalistic Primords which favoured heat like that provided by the Stahlman's ooze. (TV: Inferno)
Mawdryn and his followers stole a metamorphic symbiosis regenerator from the Time Lords and used it on themselves, only to find that their immortality was one of unending mutation and regeneration. (TV: Mawdryn Undead)