What is a pixel (px) in CSS?

Question

I am reading an ASP.NET book and it refers to CSS files and talks about pixels. But I never understood it from a resolution, layout, etc. point of view. For example, what does t

Answer 1

As stated by others, a pixel is simply a measurable unit that relates directly to the electronic display of data - a single pixel is the smallest an object on screen can be. The greater the screen resolution, the more pixels it can represent.

A note on the example given - a value of 0px is actually unnecessary as a zero value, and is better represented in CSS as just a 0 (it could equally be 0% or 0em, they all mean the same).