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variant of megalo- (megalith); also the initial element in units of measure that are equal to one million of the units denoted by the base word (megahertz). Symbol: M
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The rainy weather could not ________ my elated spirits on my graduation day.
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Also especially before a vowel, meg-.

Origin of mega-

Combining form representing Greek mégas large, great
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022


What does mega- mean?

Mega- is a combining form used like a prefix meaning “large, great, grand, abnormally large.” It is used in many scientific and medical terms.

In science, mega- is specifically used as a prefix in units of measure equaling a factor of “one million.” It’s abbreviation is M. So, a megawatt (MW) is one million watts.

Mega- comes from the Greek mégas, meaning “large” or “great.”

What are variants of mega-?

When combined with words or word elements that begin with a vowel, mega- becomes meg-, as in megohm.

Mega- is a variant of megalo- and megal-, as in megalomania and megalopsia. Learn more about their specific applications in our Words That Use articles.

Examples of mega-

A word you may have encountered that features the combining form mega- is megalith, “a stone of great size, especially in ancient construction work, as the Cyclopean masonry, or in prehistoric Neolithic remains, as dolmens or menhirs.” Some of the most well-known megaliths in the world can be found at Stonehenge.

The mega- part of the word means “large,” as we know. The second part of the word, -lith, means “stone,” from the Greek líthos. Megalith literally translates to “large stone.”

What are some words that use the combining form mega-?

What are some other forms that mega- may be commonly confused with?

Not every word that begins with the exact letters mega- or meg- is necessarily using the combining form mega- to denote “large,” e.g., megillah, megilp, or megass. Learn where these words come from and what they mean at our entries.

Break it down!

The combining form -phyll means “leaf.” What does megaphyll, which ferns and seed plants produce, literally translate to?

How to use mega- in a sentence

British Dictionary definitions for mega- (1 of 2)

/ (ˈmɛɡə) /

slang extremely good, great, or successful

Word Origin for mega

C20: probably independent use of mega-

British Dictionary definitions for mega- (2 of 2)


combining form
denoting 10 6megawatt Symbol: M
(in computer technology) denoting 2 20 (1 048 576)megabyte
large or greatmegalith
informal great in importance or amountmegastar

Word Origin for mega-

from Greek megas huge, powerful
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for mega-


One million (106):megahertz.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for mega-


A prefix that means:
Large, as in megadose, a large dose.
One million, as in megahertz, one million hertz.
220 (that is, 1,048,576), which is the power of 2 closest to a million, as in megabyte.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.