Wikipedia:
Megadeath (or megacorpse) is a term for one million deaths. It was coined in 1953 by RAND military strategist Herman Kahn and popularized in his 1960 book On Thermonuclear War. In Kahn's view, alternate scenarios in which the United States of America suffers 10 megadeaths instead of 100 should be weighed as "tragic but distinguishable outcomes." Though the term was created in order to discuss the likely consequences of conducting nuclear war, such a large number of deaths could also be associated with other weapons of mass destruction and extreme cases of democide.
In Stanley Kubrick's 1964 film Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb, General Buck Turgidson (played by George C. Scott) carries a binder titled "World Targets in Megadeaths". In the film, Turgidson promotes an all-out pre-emptive strike with the phrase "...choose between two admittedly regrettable, but nevertheless, distinguishable post-war environments...".