Flare: 1. an exacerbation of a chronic disease. Sometimes referred to as a flare-up, a flare occurs when symptoms of a disease that has been present for a time suddenly worsen. A flare is a transient worsening in severity of a disease or condition that eventually subsides or lessens. For example, in many arthritis conditions the joints can flare with worsening of stiffness, pain, and swelling. 2. An area of skin redness surrounding a site of an injection or other irritation.