What term describes the initial written or spoken single consonant or consonant cluster in a word?

The term that describes the initial written or spoken single consonant or consonant cluster in a word is called the onset. In phonetics, the onset serves as the introductory sound that begins a syllable, setting the stage for the subsequent vowel sound, which is known as the nucleus.

For example, in the word "train," the onset consists of the consonant cluster "tr." Understanding the onset is crucial in multi-sensory structured language education as it lays the foundation for phonological awareness, which helps learners decode and spell words effectively.

The other terms mentioned relate to different aspects of phonological structure: the coda refers to the consonants that follow the nucleus of a syllable, the nucleus is the vowel sound that forms the core of a syllable, and the syllable itself encompasses a complete unit of sound usually structured with an onset and nucleus, potentially also having a coda.