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'.

It's doubling each dimension. Since you're multiplying the two numbers, that turns into 4 times the number of pixels.