This is a word related in origin, as certain words in genetically related languages descended from the same ancestral word. It is also the part of the word after all affixes have been removed.

Affix

Cognates

Decode

Derivation

Prefix

Root Word

Suffix

etymology

Shown from explanation

Explanation:

How do you define the base of something: in common word formation we say words are formed combining affixes to root words. Affixes come before or after root words.

For example. Unfaithful ... 'un' is a prefix, 'ful' is suffix. Both are referred to as affixes.

Faith is the root word. So root word is that single form where an expression is tagged sensible and from it other words are derived

Etymology would well be applicable in biology.