What is Pixel pitch and how to describe it?  as we know early stage when the Film grain or granularity is described fine grain, which is told us film has higher enlargement with less grains visible on printed photographic paper.  Today digital world which is images are created on 0 1 0 1 0 1 Binary  Image depend on the type of algorithm used all in digital data form.  for example when the each color RED GREEN BLUE  layout 8 bit x 3 or Higher Bit's  represent better image quality.  for a short word each grain of the film digitally represented as a PIXEL. Gathering more pixel small area it is equal to more clear and high quality images. those film grains are described today in digital world Pixels between the gap of the pixels called Pixel Pitch..  Smaller pitch is equal to higher resolution.

Definition of Pixel Pitch  From most of  the LED Manufacturer   Alphabetical Character  P,  While we Describe P as a pitch Gap between the Pixels

For your Information Smallest Gaps represents better Image Quality,  P 1.5mm  or P 5mm or P 10mm with in the image viewing distance 2 Meter,  We Described Best,  Medium,  Poor,  if with in the image viewing distance 12 Meter.  Excellent,  Best,  Medium Above,  summing or our explanation is viewing distance important to each gaps of the Pixels.  According the viewing distance all are the actually best.