What term refers to a single letter or letter cluster that represents a speech sound?

The term that refers to a single letter or letter cluster representing a speech sound is "grapheme." Graphemes are the written symbols that correspond to the sounds in spoken language, allowing readers to decode words. For instance, the letters "b," "sh," and "tion" are all graphemes because they each represent specific sounds or clusters of sounds in words. Understanding graphemes is essential in literacy education as they form the bridge between phonics, the sounds of speech, and reading and writing.

Phonemes are the actual speech sounds, while syllables refer to units of sound within words that can contain vowel sounds and consonant sounds. Onset pertains to the initial consonant sound or cluster at the beginning of a syllable. Therefore, while these other terms are related to the study of language, they do not specifically describe the written representation of those sounds.