Mega Death (オーバーデスŌbā Desu, Overdeath?) is a recurring ability in the Final Fantasy series. It is an ability used exclusively by Yunalesca, being a more powerful version of the Death spell, used against the party with a 100% accuracy.
Megadeath is a term for one million human deaths, usually caused by a nuclear explosion. The term was used by scientists and thinkers who strategized likely outcomes of all-out nuclear warfare.
Mega is a unit prefix in the metric system denoting a factor of one million (106 or 1,000,000). It has the symbol M.