prefixes - What do you call the other bit of a word with a prefix

Let's say I have a word like unhappy.

The "un" is called the prefix. What is the other bit called?

Answer

Root noun [C] (of a word):

• The root of a word is its most basic form, to which other parts, such as affixes, can be added:
  The root of the word "sitting" is "sit".

Cambridge Dictionary