SI prefix meaning one million. Used in technical and scientific contexts.
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Colloquial intensifier meaning very large, great, or extreme. Often used in casual speech and advertising.
I hear that hotel is mega expensive.
I ordered a mega-sized ramen.
メガ is another loanword prefix used for casual emphasis (メガ盛り), but it leans toward suggesting large size or quantity, while ウルトラ emphasizes intensity or extremeness more generally.
メガ is the SI prefix for one million (10^6), while ギガ is the prefix for one billion (10^9), so they are easy to mix up when talking about data sizes or frequencies.
megawatt; MW
mega-sized portion; mega meal
megacity
megastore
megabyte; MB
megabank; major bank
megahit; blockbuster; huge success
megabit
megahertz; MHz
From English 'mega-', ultimately from Ancient Greek μέγας (mégas, 'great, large').
megavolt; MV
megamouth shark