Noun
English Wikipedia has an article on:free variationWikipedia
free variation (uncountable)
(linguistics) The situation where two or more forms appear in the same environment without a change in meaning, and without either one being considered incorrect.
They are now in free variation with f.v.t. and e.v.t. (før/efter vor tidsregning, before/after our chronology). Source: Internet
There are a few minimal pairs where the approximant and corresponding vowel contrast, but there are also many cases where they are in free variation. Source: Internet