There are several synonyms that can be used in place of the term "flare-up". One of the most commonly used ones is "outburst", which refers to sudden and intense expressions of emotion or behavior. Other synonyms include "explosion", which implies a sudden release of energy or tension, and "eruption", which suggests a sudden and violent outbreak of activity or behavior. "Frustration" can also be used as a synonym when referring to a flare-up of emotions or anger. In the context of medical conditions, "exacerbation" is often used to describe a sudden worsening of symptoms. Overall, there are many suitable alternatives for the term "flare-up" depending on the context and intended meaning.