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 whole systems. They might join together, share their ideas, and make something change. Often, revolutions include fighting, and civil unrest. But there are also revolutions that happen without fighting.
The Soviet Union was made by a revolution that killed millions, and later fell apart in a counterrevolution without much fighting. 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'.