The art or science of conveying stuff around using pipes. Sometimes metaphorically cf.Internet Plumbing.
The name derives from the Latin plumbum through OFr plomb meaning lead [ref SOED]. Nowadays lead is almost completely absent from pipework, although part of the traditional plumbing craft training is sheet leadwork.
In general 'Plumbing' refers to hot and cold water systems.
Is made from cold water by Domestic Hot Water Systems
is best for drinking
Plumbing for Gas is more commonly referred to as 'Gas Fitting' or 'Gas Installation'. It is regulated by law: specifically the Statutory Instrument known as the Gas Safety (Installation and Use) Regulations 1998