Consists of all the letters up to the vowel (e.g. initial consonants blends, and consonant digraphs). Psychologists tell us that out brains automatically separate unknown words into their onsets and rimes.
Onset refers to the beginning of a musical note or other sound, in which the amplitude rises from zero to an initial peak. It is related to (but different from) the concept of a transient: all musical notes have an onset, but do not necessarily include an initial transient.
In medicine, the first appearance of the signs or symptoms of an illness as, for example, the onset of rheumatoid arthritis . There is always an onset to a disease but never to the return to good health. The default setting is good health. See the entire definition of Onset