Tick Volume is different from the regular volume measurement in equities. In equities, each share traded is counted and thus 100 shares will equal a volume of 100. In the forex market, however, trading is decentralized and it is impossible to keep track of all the amounts and sizes of contracts in a given day. As an alternative, tick volume is measured in number of tick changes. The theory is that price movements occur in relation to trading activity and therefore Tick Volume can provinde insight into the intensity of a given price movement.
To learn more about tick volume, please visit: 120+ Technical Indicators - Volume