Bullion traditionally stands for gold bars, silver bars, other precious metals bars or ingots. The word bullion originates from the old French word bouillon, which meant "boiling" and was the term for a mint or melting house.
Bullion refers to precious metals in bulk form which are regularly traded on commodity markets. The value of bullion is typically determined by the value of its precious metals content, which is defined by its purity and mass.
The specifications of bullion are often regulated by market bodies or legislation. In the European Union, the minimum purity for gold bullion, which is treated as investment gold with regards to taxation, is 99.5% for gold bullion bars and 90% for bullion coins.
London Bullion Market 
The London bullion market is an over-the-counter market for wholesale trading of gold and silver. The London Bullion Market Association (LBMA) coordinates activities of its members and other participants in the London Bullion Market.
The London Bullion Market Association sets and promotes quality standards for gold and silver bullion bars. The minimum acceptable fineness of the Good Delivery Bars is 99.5% for gold bars and 99.9% for silver bars.
A range of professional market participants is active in the bullion markets: banks, fabricators, refiners and vault operators or transport companies as well as brokers. They provide facilities for the refining, melting, assaying, transporting, trading and vaulting of gold and silver bullion.
Besides the direct bullion market participants, other professional parties like investment companies and jewelers use bullion in the context of products or services which they produce or offer to customers. For example, shares of the world’s largest Gold exchange-traded fund, the SPDR Gold Shares represent ownership in vaulted gold bullion.
Private individuals use bullion primarily as an investment or potential store of value. Gold bullion and silver bullion are the most important forms of physical precious metals investments. Bullion investments can be considered as insurance against inflation or economic turmoil and may not entail direct counterparty risk.
Compared to numismatic coins, bullion bars or bullion coins can typically be purchased and traded at lower margins and their trading prices are closer to the values of the contained precious metals.
- Merriam-Webster Dictionary: Bullion - Definition and More, http://www.merriam-webster.com/dictionary/bullion
- Council Directive 98/80/EC of 12 October 1998 - Special scheme for investment gold: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31998L0080:en:HTML
- The London Bullion Market Association: Glossary of terms - Bullion and precious metals coins, http://www.lbma.org.uk/pages/index.cfm?page_id=89&title=glossary_of_terms#B
- A Guide to the London Precious Metals Markets: http://www.lbma.org.uk/assets/OTCguide20081117.pdf
- Przemyslaw Radomski, CFA. "Key Insights: Portfolio Structure". Sunshine Profits. Retrieved 2012-11-15.