Between them you have : http://www.oxfordreference.com/view/10.1093/oi/authority.201...
1 A market that is free from government interference, prices rising and falling in accordance with supply and demand.
2 A security that is widely traded on a stock exchange, there being sufficient stock on offer for the price to be uninfluenced by availability.
3 A foreign-exchange market that is free from influence on rates by governments, rates being free to rise and fall in accordance with supply and demand.
So, I can see why that was used, but it's not definitive.