What is the term used for a prefix or suffix added to a root or base to form another word?

The term for a prefix or suffix added to a root or base to form another word is indeed derivational affix. This type of affix is crucial in word formation as it results in the creation of a new word with a meaning that can differ significantly from the original root. For example, adding the suffix "-ness" to the adjective "happy" creates the noun "happiness," which embodies a distinct meaning.

Derivational affixes can change the part of speech of the root — turning verbs into nouns or adjectives, for instance. This transformation is a key element in language development and vocabulary expansion, allowing for greater expressiveness and specificity in communication.

In contrast, inflectional affixes serve a different purpose by modifying a word to express grammatical features like tense or plurality rather than creating new words with unique meanings. Other options, such as compound suffix and conjugative prefix, do not accurately encompass the concept of a general affix used for word formation in the same manner as derivational affix does.