What Word?

Definition: “An item in a list of "possibilities", usually a set of extractor errors. A ___________ consists of a message, and a set of parts that belong to the same def. Each ___________ can be visited independently; the system keeps track of which ones have been seen. [might be good to invent better nomenclature. -- rgr, 18-Jan-03.]”

First letter: P

Answer:  

Which word is being defined?

"IQ" Score: 0