1.A Prison where you are allowed to leave during the day to go to a place of employment, the pub, score crack or just visit the local children's playground.
2.A term used to describe a Pontins Holiday parks. However, open prisons as described in 1 are probably less grubby and better maintained. They are also frequented by fewer petty criminals.
"I've just been on a mini-break to Pontins near Burnham-on-Sea. Would have had a better time at Strangeways"
"I've heard it's a bit of an open prison"