:Extrinsic allophone:

An allophone of a phoneme which cannot be explained simply by reference to phonetic features of the environment in which it appears. An example is the velarised allophone [ɫ] of the /l/ phoneme in many accents of English. This appears in all environments except immediately before a vowel or j. There is nothing common in the environments in which it does occur which could explain the velarisation.

Compare intrinsic allophone.