SI prefix meaning one million. Used in technical and scientific contexts.
このファイルは5メガバイトあります。
This file is 5 megabytes.
Colloquial intensifier meaning very large, great, or extreme. Often used in casual speech and advertising.
あのホテルはメガ高いらしいよ。
I hear that hotel is mega expensive.
メガ盛りのラーメンを注文した。
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