The commonly used term for illegally organised raves, usually held in some skanky derelict building/warehouse (but can be outside too) and located by calling a given mobile number after a specified time on the night of the party. Normally has a start time but no end time as it generally goes on until the police get wind and shut it down! Usually a VERY messy affair involving a blend of acid techno/psytrance copious amounts of hedge monkeys and a side order of random chavs.
"I went to an amaaazing free party in that office block last night! Everyone was lovin it til the rozzers shut it down!!"
de carlypussycat 13 Martie 2008