PO-1

PO-1 This is for "Poor" (the grade) and "1" (the numerical designation that means Poor).

Poor (PO-1)
Poor coins are essentially uncollectible. They are completely or almost completely worn down to the point of being unreadable.
Fair (FR-2) ...

The grade PO-1. A coin with readable date and mint mark (if present), but little more, barely identifiable as to type. (One-year type coins do not require a readable date to qualify for this grade.)
porous ...

Fair-2 (FR2): Just slightly more detail than PO-1. Much of the lettering and detail will be worn away
About Good-3 (AG3): This coin is flat with little detail remaining and with the rims worn down into the lettering.

Poor
Synonym for the grade PO-1 (or P1).
Pop Report
Synonym for a report issued by NGC or PCGS, which summarizes the coins certified to date.

See also: Grade, Coin, Poor, PCGS, Half