Definition of "flare-up"

flare-up

noun
plural flare-ups
1. A sudden outbreak, outburst or eruption (originally of flame, but now used more generally of any violent activity or emotion).
  Quotations
  But what I do think is that there's likely to be a regular flare-up soon. And how can one tell where that may end? If only I could get Ambrose to take me away somewhere! But he won't budge.
  1933, Richard Curle, Corruption, page 75
  The disadvantage of the underfired grill is the potential for flareups, which can cause a burned spot or carbonization. Not only do these taste bad, but they are also carcinogenic.
  2009, Peter Osbaldeston, The Palm Springs Diner's Bible: A Restaurant Guide for Palm Springs, Cathedral City, Rancho Mirage, Palm Desert, Indian Wells, la Quinta, Bermuda Dunes, Indio, and Desert Hot Springs, Gretna, La.: Pelican Publishing Company, page 250
2. A sudden intensification in activity.
3. An inflammation such as of tendons (tendonitis) or joints (osteoarthritis).
  Quotations
  Although Crohn disease and ulcerative colitis are chronic, they are characterized by periods of remission that last for varying amounts of time, interrupted by flare-ups, when symptoms of the disease become more evident.
  2000, Jon Zonderman, Ronald Vender, Understanding Crohn Disease and Ulcerative Colitis‎