What is an affix in linguistic terms?

An affix in linguistic terms is correctly identified as a morpheme attached to a root word. This means that affixes serve as building blocks that modify the meaning of the root word when they are added. Affixes can be prefixes, which are added to the beginning of a word, or suffixes, which are added to the end. They can also include infixes and circumfixes in other languages.

When affixes are employed, they can alter the grammatical function or the meaning of the root word. For example, adding a suffix like "-ing" to the verb "run" creates "running," indicating ongoing action. In this way, affixes are essential in expanding vocabulary and facilitating nuanced expression within a language.

The other options provided do not accurately define an affix; phrase structure pertains more to syntax and sentence structure, phonetic pronunciation relates to how words are spoken rather than their morphological structure, and the language used in sign communication describes a completely different modality of language that does not directly involve the concept of affixes in spoken or written language.