Policy enshrined by the old Clause IV of the British Labour Party's constitution, and carried out by Labour governments (see British Coal, British Nuclear Fuels, British Telecom, British Gas, British Airways, British Rail, think of an industry and stick 'British' in front of it).
The opposite of privatisation, it's basically a Marxist policy whereby the state buys and owns the industry, and each state-owned company has a monopoly in their particular area. Public services, therefore, are supplied by the government, and are not simply more fodder for capitalism (communist? me?).
The downside to all this, according to the right wing, is that the lack of competition allows prices to rise and quality to fall. It was with this rationale that Margaret Thatcher and John Major brought in a policy of privatisation, which was continued by Tony Blair and his half-assed plans to privatise the London Underground and air traffic control.