Transmutation was the changing of something into something else unlike its prior form. This could also be referred to as metamorphosis, particularly when speaking of a living organism.
In 2268, Spock noted that neither the USS Enterprise's or Klingon technology was capable of the "the instantaneous transmutation of matter", concluding that another force other than either of the two was responsible for the changing of phasers into swords. (TOS: "Day of the Dove")
In 2366, John Doe described his species's change into non-corporeal lifeforms as "a transmutation beyond our physical being." (TNG: "Transfigurations")
In 2372, while changing into a reptilian form, Tom Paris underwent neuroelectrical transmutations, according to The Doctor. (VOY: "Threshold")