Answer:
a post or email message that expresses a strong opinion or criticism.
Step-by-step explanation:
The term "Flame" start to arise after the rise of internet communities.
Most of the interactions that people made through the internet are anonymous. So, many people felt that they can get away with saying something hurtful through unthoughtful opinion or criticism without facing the deprecations.
The purpose of such message is usually to ignite anger. Which is why the term 'Flame' is often used to describe this type of behavior.