Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
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 *meg- "great" (cognates: Latin magnus, 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]
pre. 1 (context originally English) Very large, great. Denoting a size larger than usual. 2 In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by one million (106.) SI Symbol: M. 3 (context computing English) Multiplying the unit to which it is attached by 220 (= 1,048,576, the binary round number closest to a million). Computing symbol: Mi. 4 (context computing, marketing English) Multiplying the unit to which it is attached by 210 × 103 (= 1024,000, the binary round number closest to thousand).
Mega (also spelt Méga) is a unit prefix in the metric system denoting a factor of one million (10 or ). Symbol (M). It was confirmed for use in the International System of Units (SI) in 1960. Mega comes from the Greek , meaning "great".