From Diablo Wiki
Spiky damage is damage that comes in big blasts, rather than a steadier stream. The term generally refers to damage taken by characters, and avoiding spiky damage (and one-shot kills) was a main goal of the Diablo 3 developers as they reworked the game systems and combat for Diablo 3 version 2 and Reaper of Souls.
Their effort was largely successful, and combat in RoS is more strategic and balanced, with characters able to use various sorts of mitigation to keep incoming damage to a manageable level.
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