mega-

WordReference Random House Learner's Dictionary of American English © 2026

mega-, prefix. 

1. mega- comes from Greek, where it has the meaning:
   • extremely large, huge:
     megalith (= extremely large stone or rock); megastructure (= a huge structure).
   • mega- is also used to mean one million of the units of (the base root or word):
     megahertz (= one million hertz); megaton (= one million tons).
   • mega- also means very large quantities or amounts:
     megabucks (= a great deal of money); megadose (= a large dose of medicine).
   • mega- also means things that are extraordinary examples of their kind:
     megahit (= a smash movie or stage hit); megatrend (= important, very popular trend).

WordReference Random House Unabridged Dictionary of American English © 2026

mega-, 

1. var. 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 

Also,[esp. before a vowel,] meg-. 

• combining form representing Greek mégas large, great

Collins Concise English Dictionary © HarperCollins Publishers::

mega- combining_form

1. denoting 10⁶: megawatt
   Symbol: M
2. (in computer technology) denoting 2²⁰ (1 048 576): megabyte
3. large or great: megalith

Etymology: from Greek megas huge, powerful