Which term defines a letter or letter combination that spells a single phoneme?

The correct term that defines a letter or letter combination that spells a single phoneme is "grapheme." Graphemes are the written representation of phonemes, which can be a single letter (like 'b' for the phoneme /b/) or a combination of letters (such as 'sh' for the phoneme /ʃ/). Understanding graphemes is crucial for reading instruction, as they form the foundation for decoding words.

Other terms like phoneme refer specifically to the smallest unit of sound in spoken language but do not encompass the written aspect. Diphthongs are specific types of vowel sounds that begin with one vowel sound and glide into another within the same syllable, and blends consist of two or more consonant sounds that are pronounced together, but they don't directly relate to graphemes.