Z-axis
From Guild Wars 2 Wiki
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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, allow characters to jump over obstacles, and carry out other actions enabling a deeper interaction with the environment, such as sliding and swimming.

