onset

Onset is a term used in linguistics to designate the initial consonant or consonant cluster of a syllable, preceding the syllable nucleus (usually a vowel). The combination of onset, nucleus, and coda is often described as the syllable, with the rhyme consisting of nucleus and coda. Some languages allow a null onset (no consonant before the vowel); others require an onset. Examples: in the word street, the onset is str; in banana, the onset of the first syllable is b; in the word eat, the onset is null.

In phonology, onset is the segment that begins the syllable's sonority peak; onsets vary across languages due to phonotactic constraints. The concept helps explain syllable structure in different languages, and some theories assume onset maximization, where speakers prefer to attach consonants to the onset when possible.

Onset also has a meaning in music and acoustics. It denotes the moment when a sound or note begins, i.e., the attack. In auditory science and signal processing, onset time or onset detection identifies the start of events in an audio stream, and is used in rhythm analysis, tempo tracking, and instrument recognition.

In medicine and epidemiology, onset refers to the time at which symptoms or signs begin. Onset can be described as acute or gradual, and clinicians distinguish onset time from duration and course. Onset latency describes the delay between a trigger and the appearance of symptoms, such as the onset of fever after infection or the onset of drug action.

Usage in general language extends to phrases like onset of an epidemic, onset of winter, or onset of action describing when a drug begins to have an effect.