A Pixel is the smallest unit that can be addressed on the screen. On Windows and Linux, a custom screen resolutions is 1024x768 pixels. This means, the entire screen is created from 1024 columns and 768 rows, summing up to 786432 single pixels.
Pixel 0,0 would be in the top left corner, Pixel 512,382 would be one of the four pixels in the center of the screen and Pixel 1023,767 would be the bottom right.
On a 15" Monitor, a single pixel would measure about 0.3 millimeters on the screen.