It's called '2x' resolution because it's twice as many pixels per dimension, and because you append "@2x" to image resources for them to be used in your app. It

It's called '2x' resolution because it's twice as many pixels per dimension, and because you append "@2x" to image resources for them to be used in your app. It is 4x the pixels, but that I guess Apple figured that took slightly more brainpower than just imagining twice as many pixels tall, and twice as many pixels wide and calling it '2x'.