Noun
P-box (plural P-boxes)
Abbreviation of permutation box.
A good P-box has the property that the output bits of any S-box are distributed to as many S-box inputs as possible. Source: Internet
A permutation box (P-box) is a permutation of all the bits: it takes the outputs of all the S-boxes of one round, permutes the bits, and feeds them into the S-boxes of the next round. Source: Internet