KDE Visual Design Group/HIG/Units
This pages gives an overview about different units used in KDE and Plasma
A (physical) pixel is a physical point in a raster image, or the smallest addressable element in an all points addressable display device. Be careful not to confuse it with PPI independent pixels
PPI - pixels per inch
Pixel density is the number of (physical) pixels that fit into an inch. Different screens have different PPI.
screen density = screen width (or height) in pixels / screen width (or height) in inches
PPI is often used interchangeably with DPI, dots per inch.
PPI independent pixels
A PPI independet pixel is scaled to look uniform on any screen regardless of the PPI. A lot of platforms, eg iOS, Android, the Web, replaced the old (physical) px with a PPI px. So most the time you read about pixel/px they actually talk about PPI independent pixels. Qt (and QML) support PPI independent pixels in newer versions, but because KDE and Plasma support older versions of Qt too, one can not assume that pixels used in Qt or QML apps are ppi independent.
Note to developers: Except explicilty stated otherwise, all HIG pages, draft, mockups, ... pixels/px are always PPI independent pixels.