adjectival noun or quasi-adjective, takes genitive case の, noun

Usually written with kana alone.
  1. sudden, abrupt, unexpected, improvised, hasty, offhand.

noun

Abbreviation.
  1. impromptu skit. Sometimes 仁輪加 (ateji).
  2. bandwagon fan, fair-weather fan. Slang, usually written with kana alone.

Variations

Pronunciation Kanji Kana Is Common
 
 
 
にわか
にわか
 
俄か にわか