What term defines a meaningful part of a word that is attached to a root or base word?

The term that defines a meaningful part of a word that is attached to a root or base word is an affix. An affix is a general term that encompasses both prefixes and suffixes, which are specific types of affixes. Prefixes are added to the beginning of a root word, while suffixes are added to the end.

Understanding the function of an affix is crucial in analyzing how words are formed and how their meanings can change. For instance, adding the prefix "un-" to the root word "happy" changes the meaning from "happy" to "unhappy," demonstrating the significant role affixes play in word formation.

In contrast, a morpheme refers to the smallest unit of meaning in a language. While all affixes are morphemes, not all morphemes are affixes, as some can be whole words. A root is the core part of a word that carries its primary meaning, and while it can stand alone as a word, it does not include the limitations of being defined as an affix. Thus, affix is the most appropriate term to describe a meaningful part attached to a base or root word.