Volume is : the indication of supply and demand. It’s defined as the number of units traded during a time period. This number is significant in that it supports the prevailing price trend.
Volume should expand in the direction of a major trend. If the trend is up, the volume should increase as buying pressure exceeds selling pressure. As prices are accepted and level off, there will also be a levelling in volume. The same would be true if prices fall. Volume will expand, representing a major change in the trend.
Example :
The following chart shows Merck and its volume.





