derivative word; derived word
Linguistics term for a word formed from another word, e.g., by adding a prefix or suffix.
"Kanashimi" is a derivative of "kanashii".
This dictionary also lists derivatives as headwords.
語根 is the root or base morpheme from which a 派生語 is derived.
派生 (derivation) + 語 (word). A straightforward compound of Sino-Japanese elements.