Trading Session

In New York, a gun goes off (ok, it's a buzzer or a bell) at 9:30 New York time. The trading session starts. Then, at 4 pm, the buzzer or bell hits again, and the trading session ends. That's a trading session at the NYSE.

But stocks trade all the time, all around the world, 24x7x365 in one form or another. (See: ECN.) So there is no such thing as a "globally attributable" trading session. Sessions only exist in context in narrow forms, on specific exchanges. And that's it.