[noun] [U] - the moment at which something that is usually unpleasant beginsThere are usually several years between exposure to the virus and the onset of illness.The new treatment can delay the onset of the disease by several years.The negotiators have given up hope of signing a peace settlement before the onset of winter.
ONSET
Meaning of ONSET in English
Cambridge English vocab. Кембриджский английский словарь. 2012