Which type of morpheme is defined as a bound morpheme that alters words but does not stand alone?

A bound morpheme that alters words but cannot stand alone is best defined as an affix. Affixes are morphological units that attach to a root word or stem to modify its meaning or create a new word. For instance, prefixes and suffixes are types of affixes that are used to alter the base meaning of the word they are attached to.

In contrast, free morphemes can function independently as words on their own, while roots are the core components of words that typically carry the primary lexical meaning. Compounds involve the combination of two or more free morphemes to create a new word, but they do not pertain to the concept of bound morphemes. Therefore, the definition of an affix appropriately captures the idea of a morpheme that relies on a base and alters it, embodying the essence of what makes these bound morphemes integral to word formation.