A syllable onset is the constituent preceding the syllable nucleus within a syllable. It may contain one consonant or a consonant cluster, and some syllables have no onset. A language's phonotactics constrains which combinations can occupy that position.[ref]
Essentials of Linguistics (2nd edition) by Catherine Anderson and others. §3.10, syllable margins, onset clusters and onsetless syllables
In “cat”,
[k] is a simple syllable onset; in “stay”, [st] forms a cluster before the vowel. A vowel-initial pronunciation can instead begin directly with the syllable nucleus. This is a statement about spoken structure: the number of written initial letters need not equal the number of onset consonants, and no physical pause is required at every syllable boundary. New to topics? Read the docs here!