A revolution is a very sharp change made to something. The word comes from Latin, and is related to the word revolutio (which means a turn around).
Revolutions are usually political in their nature. Some people feel unhappy with their lives, some are not happy with the whole system. They might join together, share their ideas, and make a revolution. Often, revolutions include fighting, and civil unrest. But there are also revolutions that happen without fighting as well.
The Soviet Union fell apart into its different republics without much fighting in 1990. But in the French Revolution (1789), there was much bloodshed. The years right after this Revolution in France are often called the Reign of Terror.
Other events often called "revolutions" include:
The opposing idea in politics is called 'gradualism'.