The number of shares or contracts traded in a security or an entire market during a given period of time. It is simply the amount of shares that trade hands from buyers to sellers as a measure of activity. If a buyer of a stock purchases 1000 shares from a seller, then the volume for that period increases by 1000 shares based on that transaction.
Volume is an important indicator in technical analysis as it is used to measure the worth of a market move. If the markets have made a strong price move either up or down the perceived strength of that move depends on the volume for that period. The higher the volume during that price move, the more significant the move.