it is Liquefied Petroleum Gas.
LPG, or propane, is widely-sought after for use in BBQ's, caravans, patio heaters, and catering wagons. It is sold in red bottles and is supplied by Calor although many others offer it. It's not the same as the other natural gas commonly used for cooking. Most propane gas conversions in the UK are done using liquid propane. You can use Propane if you live in a warmer climate or if you want to use it during the warmer months.