Volume is the number of contracts, shares or any other unit of trade in a security over a certain period of time. Trade volume can be measured on any time frame but is most often done on the hourly, daily and weekly basis. A buyer and a seller must be involved in the transaction to constitute a change in volume.
In the world of technical analysis, volume is a very useful tool. For example, when volume is slowing over a series of bars while underlying price action continues to move higher, it may indicate that the existing level of bullish interest is fading and a reversal is on hand.
Assets like stocks, futures and bonds are often dealt on exchanges or through centrally networked OTC markets, where volume is more easily tracked. Foreign Exchange on the other hand is traded on the Interbank Market, a network of thousands of banks. Because FX has many different points of trade, volume is nearly impossible to track.
To remedy this, variations on traditional volume indicators are used. Tick volume measures each tick higher or lower in price action as a unit of trade. This measurement side-steps the problem of being unable to track actual trade, but it also fails to account for offsetting trades that occur at the same time.