:Onset:

(1) The first pre-nuclear accented syllable in an intonation pattern. The stretch of the intonation pattern from the onset to (but not including the nucleus) is known as the head.

(2) A component of the syllable, consisting of any consonants which precede the syllable nucleus. The onset may be empty in some (but not all) languages. In the English word split the onset is the cluster of consonants spl .