Liquidity is a measure of trading interest in a market. It is defined as the ability to sell an asset without affecting the price or value of the asset. FOREX is the biggest and most liquid market in the world as of November 2010, with almost $4 trillion of daily activity. Nonetheless, liquidity can vary substantially, especially during economic shocks. There are several methods available to gauge the liquidity of any pair of currencies. The most popular currency pairs, such as euro/dollar and dollar/yen, are the most liquid.
http://www.forex-metal.com/affiliate/47225/1