Two or more countries with a single currency. Countries which formerly had independent monetary systems unite to adopt a single currency, or keep separate currencies but enter into a permanant and credible agreement to maintain a constant exchange rate between their currencies. A monetary union requires either a single central bank or the adoption of effective policy co-ordination between the central banks of the member countries; the Euro zone in fact has both.

• over-valued currency
• owner-occupied housing
• ownership
• ozone layer
• Paasche index
• package of policies
• paid-up capital
• panel data
• paper
• parabola
• paradox of thrift
• paradox of voting
• parameter
• Pareto-optimality
• Paris Club
• parities, Smithsonian
• parity, purchasing power
• partial adjustment
• partial derivative
• partial equilibrium
• participation rate
• partnership
• part-time work
• par value
• patent