mega- Look up mega- at
before vowels meg-, word-forming element often meaning "large, great," but in precise scientific language "one million" (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE root *meg- "great" (source also of Old English micel; see mickle). Mega began to be used alone as an adjective by 1982.
High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]