Z-axis
From Guild Wars 2 Wiki
The Z-axis is the axis in three dimensions relating to the vertical, pointing up and down from the horizontal.
Although mostly ignored in the original Guild Wars game series, Guild Wars 2 will introduce recognition of the Z-axis to the game which will prevent exploits such as being able to melee attack someone on a bridge by standing under the bridge. It will also allow characters to jump over obstacles and carry out other actions such as sliding and swimming, enabling a deeper interaction with the environment.
