Hexspine: Definition

Term used in wargaming to designate a side that is common to two adjacent hexagons on a map.

Hexspine, or hex spine, is most often used when evaluating line of sight. For example, wargames will typically address cases where the line of sight goes down the hexspine between two adjacent hexes that have different types of terrain.

So a Wall hexside that is not one of the six sides of the sighter’s or target’s hex blocks LOS between them, even if the LOS is traced exactly along the Wall’s hexspine.Combat Commander: Europe Rulebook