A nickname for the Federal Bereau of Investigation. This was a term coined through several popular image boards, particularly 4Chan, following a comic strip that depicted an anonymous user opening his door after being told that there was a party van outside. Immediately afterwards he is attacked by several agents. The term is often used as a warning against illegal activities on the internet.
"Don't answer the phone!"
"It could be the partyvan checking to see if you're home!"