"a" and "an" - What is the name for adding characters for better pronunciation?

We say "a while" but "an hour". The n seems to be added for better pronunciation.
What is the name for this linguistic construct (adding characters to improve pronunciation)?

In French, latent word-final consonants are pronounced before a vowel sound which is known as "Liaison" but this seems to be the wrong term for the addition of characters in English.