Which term is used to describe the sound units formed by a single letter or combinations of letters?

The term that describes the sound units formed by a single letter or combinations of letters is "phoneme." Phonemes are the smallest units of sound in a language that can distinguish one word from another. For example, the words 'bat' and 'pat' differ in only one phoneme, which changes the meaning of the words entirely.

In contrast, morphemes refer to the smallest units of meaning, which can be single words or parts of words (like prefixes or suffixes). Graphemes are the written representation of phonemes - they correspond to the letters or letter combinations used in writing. Syllables are larger units of sound that typically contain a vowel sound and may include surrounding consonant sounds. Phonemes are thus fundamental to the understanding of spoken language, as they help learners decode and pronounce words correctly.