Which term describes the sound structure of language, involving the smallest units?

The term that describes the sound structure of language, involving the smallest units, is phonological units. Phonology focuses on the way sounds function in particular languages and how they are used in communication. The smallest units in phonology, known as phonemes, are crucial for understanding how different sounds can change meanings in words. For example, the change of a single phoneme can alter the meaning of a word entirely, such as "bat" versus "pat."

Etymology pertains to the origin and historical development of words, which does not focus on sound structure. The lexicon refers to the vocabulary of a language, including the words and their meanings, rather than their sound components. Syntax involves the rules and structure of sentences in language, governing how words combine to form phrases and sentences, but does not deal specifically with the smallest sound units. These distinctions highlight why phonological units accurately define the sound structure aspect of language.