Mega- is a prefix meaning “extremely large” or “one million.” When you attach it to the front of another word, it amplifies that word's meaning in a big way.

In everyday language, mega- means huge or impressive. A megastar has achieved spectacular fame and success. A megastorm packs tremendous power. When something goes mega-viral online, millions of people see it. Sometimes people use mega by itself as slang for “really” or “very,” as in “That roller coaster was mega fun!”

In science and math, mega- precisely means one million. A megabyte is often used to mean about one million bytes of computer data. A megawatt equals one million watts of electrical power. Scientists use these standard prefixes to describe quantities too large to count easily. When you hear about a megacity, you're hearing about a very large urban area, often with millions of people.

Other prefixes work the same way: kilo- means one thousand, while giga- means one billion. Understanding these prefixes helps you grasp the scale of numbers that would otherwise be hard to imagine, whether you're measuring computer memory, electrical power, or the population of Tokyo.