free variation
noun Linguistics.
a relation between the members of a pair of phones, phonemes, morphs, or other linguistic entities such that either of the two may occur in the same position with no change in the meaning of the utterance: in the first syllable of “economics,” “e” and “ē” are in free variation.
Compare complementary distribution.
Dictionary.com Unabridged
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019