Flare-up

A flare-up or flare or symptoms flare in myalgic encephalomyelitis/chronic fatigue syndrome is a temporary increase in symptoms, alternatively known as experiencing post-exertional malaise. Some people use "crash" or "collapse" to refer to a flare.

NHS definition
The UK's National Health Service definition of a flare-up can be found within the 2021 NICE guidelines.

International Consensus Criteria
Symptoms flare is not a term defined in the ICC.