Hex
From AoP Wiki
A Hex is a measurement in the game. Like a chess board divided into squares, the game has a grid of hexagons. This means a character has 6 directions to walk to at a given moment (unless of course hindered by an obstacle).
Distances are measured in Hexes. Speed in Hexes per second.
An entity who can walk 3 tiles per second, thus has a speed of 3 hexes per second.
A weapon that shoots up to 20 hexes, can shoot at any point that is reachable by walking 20 tiles in a direction. (Given, it is an open field, without obstacles)