Short answers, pulled from the story.
A world war is an international conflict involving most or all of the world's major powers. Conventionally the term refers to World War I (1914-1918) and World War II (1939-1945), though historians have also applied it to the Seven Years' War, the Cold War, the war on terror, and several other large-scale conflicts.
The first known use of the term in the English language appeared in a Scottish newspaper, The People's Journal, in 1848. Karl Marx and Friedrich Engels also used the phrase in articles published around 1850.
Time magazine coined "World War I" on page 28 of its issue dated the 12th of June 1939, and used "World War II" speculatively on page 32 of the same article. The Danish newspaper Kristeligt Dagblad was the first to use "Second World War" for the actual conflict, running it on its front page on the 4th of September 1939.
Estimates for World War I range from 15,000,000 to 65,000,000 casualties. World War II casualty estimates range from 40,000,000 to 85,000,000. Both figures include combatants and noncombatants.
World War II is the only conflict in which nuclear weapons have been used in combat; the United States dropped atomic bombs on Hiroshima and Nagasaki. The war also included the Holocaust, in which approximately six million Jews and about five million others were murdered by Nazi Germany, and the Nanjing Massacre, in which 250,000 civilians were killed by Japanese troops.
Einstein is often quoted as having said in 1947: "I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." The remark reflects widespread fear, following the atomic bombings of Hiroshima and Nagasaki, that a nuclear third world war would be catastrophic beyond comparison.