Answer 1

Pixels can be measured: they are unit, wich represent the smallest "PICture ELement"... where the picture is a grid of pixels ( the well known bit-map ) A ( physical ) display have a physical size ( in "human" size, like inch, centimeter... ) and a resolution/logical size ( in pixels : number of pixels in width and in height -- the size of the pixels grid ). So, the pixel can be me measured with the formula: physical size / logical size...

Well, another way to get a measure of pixel, is to calculate his density: how many pixels are in a particular size... the well heard dpi unit ( Dot Per Inch -- but other units ratio are possible ), from which we ca also calculate the physical size of a pixel, by doing: for density in dot per unit ratio, size of pixel equals 1 / density in unit...

However, pixels are not ever pixels, since mobile devices ^^
Look at this web article about high resolutions images and responsive design: https://www.html5rocks.com/en/mobile/high-dpi/ It introduce with explanation about why and how the special "pixel css" was introduce a new pixel type.