Which type of affix is defined as a morpheme that precedes the root or base word?

A prefix is a specific type of affix that comes before a root or base word, altering its meaning. For example, in the word "unhappy," the prefix "un-" is added to the root word "happy" to convey the opposite meaning. This distinction is important in understanding word formation and the construction of vocabulary in English.

While affixes can also include suffixes, which follow the base word, and other elements like roots themselves—none serve the same function as prefixes. A root is the core part of a word that carries its primary meaning, whereas an affix is an additional morpheme that modifies that meaning. Thus, the definition provided in the question aligns perfectly with the concept of prefixes.