The number of shares transacted every day. As there is a seller for every buyer, one can think of the trading volume as half of the number of shares transacted. That is, if A sells 100 shares to B, the volume is 100 shares.
Automated Pit Trading (APT) Introduced in 1989, APT is the LIFFE
screen-based trading system that replicates the open outcry method of trading
on screen. APT is used to extend the trading day for the major futures contracts
as well as to provide a daytime trading environment for non-floor trading
products.