Mutation

Mutation is a process by which a new word is formed without affixation, but simply by a change of the initial consonant of the base.

Example
in the Siberian language Nivkh (or Gilyak) nouns can be derived from verbs simply by changing the initial consonant (see (i)), and likewise transitive and intransitive verbs are often related solely by mutation (see (ii)):

(i) vutyidy 	'sweep'			putyis	  'broom' fady	 'put on knee-piece'	phady	 'knee-piece' (ii) gesqody	'burn NP'		kesqody 'burn oneself' zody	'bend'			tyody  'bend' (Cy = palatalized C, Ch = aspirated C)

Links
Utrecht Lexicon of Linguistics