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

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

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.

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.