It is a type of anticipatory (or regressive) assimilation at a distance.
The vowel that triggers the change was later normally lost.
In Welsh, gair "word" and -iadur "device suffix" yield geiriadur "dictionary", with -ai- in gair becoming -ei-.
The two main types of affection are a-affection and i-affection.
More rarely, the term "affection", like "umlaut", may be applied to other languages and is then a synonym for i-mutation generally.