Loosely, the G5 was a group of 5 major industrialised countries comprising France, Germany, Japan, the United Kingdom and the United States.
Largely superseded by the G7.
More strictly, a meeting of the heads of government or the heads of central banks from those countries.
Also known as the Group of Five or 'G-5'.
The G5 is also a group of 5 developing countries comprising Brazil, China, India, Mexico and South Africa.