Revolution

Revolution is the overthrow and replacement of government. This can occur violently (like most of the ones we usually talk about) or non-violently (like the many "color" revolutions in the early 21st century.) Revolutions are usually swift; if they aren't, they're usually referred to as a civil war. (The exception is when colonies try to throw off their colonial masters; those are still called often called revolutions, even when the fighting goes on for decades.)

The French (1789 – 1799) and Russian (1917) revolutions are the most noteworthy although there was a minor altercation in North America (1775 – 1783) which has some claim to notoriety.

A counterrevolution occurs whenever someone tries to undo the revolution. It is an odd thing, though, when powers that have been in place for 50 years continue to repress people due to fighting "counterrevolutionaries." At that point, you'd think they'd be regular revolutionaries again.

Other uses

A revolution can also refer to a major change in society, such as the Industrial Revolution which led to mass production, or the Green Revolution which brought efficient forms of agriculture to the Third World, or the Information Revolution which brought all of human knowledge to the fingertips of every well-off western man, woman, and child on Earth.