panchromatic

This term is not so much used nowadays in the digital age.  It refers to black and white film that has a flat response to all wavelengths of light and so reproduces images that look more like the human eye's perception.  Earlier orthochomatic film produced the wrong grey tones for certain colours.

Pixel

This stands for Picture Element and is explained in topic 1.

PNG (Portable Network Graphics)

One of the many graphics file formats.   Also explained in topic 1.

positive

The opposite of negative q.v.

PPI (Pixels Per Inch)

Only relevant to printing, not dealt with further here.