Does a "backlash" mean a repercussion ? Or a violent reaction ?
The sentence is " The stunt has provoked a social media backlash"

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Answer

Yes , its a reaction that that is adverse or not good for the recipient

Answer

A "backlash" is an example of a "repercussion."  Repercussion is a reaction that results from an action. They might be strong or weak but are almost always negative. Backlash is a strong negative reaction.

To include both words in one sentence: the repercussion to the stunt was a strong social media backlash.