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?

Solution 1:

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

Solution 2:

Stem noun:

• the part of a word that is left after you take off the ending

Although I'd disagree with that and allow "taking off the prefix" too, as per word stem:

• a stem is a form to which affixes can be attached

See also affix, suffix, infix, adfix.

Solution 3:

This may be the math talking, but the word 'base' is commonly used to describe the word that a prefix or suffix can be added to.

the root or stem of a word or a derivative.

• the uninflected form of a verb.

-Oxford [2010]

I would like to note that (take this maths) that derivative would not quite work, as it means

a word derived from another or from a root in the same or another language.