In CSS context, a screen is a (visual) media type that is intended primarily for color computer screens.
A user agent on a screen generally impose a minimum width and choose an initial width based on the dimensions of the viewport.
See also UI - Screen
The CSS resolution is always given in Css Pixel (not in Hardware pixel)
The CSS screen resolution is determined by:
Example:
<MATH> \text{Maximum Width in CSS Pixel} = \frac{\displaystyle 1920}{\displaystyle 2} = 960 </MATH>
Why would the same page look different on the screens with identical resolutions ?
In Google Analytics, you can find the screen resolution at Audience > Technology > Browser > Screen Resolution