From Teflpedia

Derivation /ˌde.rəˈveɪ.ʃən/ involves changing a word, often by adding an affix (prefix or suffix) to a word, thereby changing the meaning of the word.

For example, in English, undertake, overtake, intake, takeover, etc are all derivations of take.

Contrast inflection where the lexical meaning of the word is unchanged.

If the word is unchanged, it's called conversion or zero derivation.