Melee
Melee describes that part of combat in which hand-to-hand combat takes place. Combat is resolved with the assumption that both combatants are simultaneously engaged; only the order in which blows are given and received is turn-based. It is therefore assumed that, once combatants enter adjacent hexes with one another, melee has occurred and neither may easily extricate themselves. In the diagram shown, Albert and Bala are not in melee. They may hurl things at each other, but it is presumed for game purposes that they are not engaged in hand-to-hand combat.
However, if Albert moves into any of the melee hexes surrounding Bala (tinted yellow), this establishes melee. Albert can avoid doing this by moving into 0104 and then into 0205, without engaging. We might imaging the combatants moving in wide circles around each other. However, if he enters any of the yellow-tinted hexes, then Albert has established melee and is considered to be defending himself, even if it is not Bala's "turn." Nor does it matter if matter if Albert has enough movement to attack; by moving into an adjacent hex, Albert also forces Bala into melee.
To withdraw at that point, he must pay a movement penalty of 1 action point (AP), in addition to the normal cost of movement, to move out of that melee hex. If he does not have enough movement to do this, he must remain and allow Bala to take an action. Bala is in the same circumstance; she must also now spend a movement penalty to leave the hex now that Albert has engaged her.
Note that this movement penalty applies to the hex being moved out of, not into. It would not matter whether Albert were moving into another combat hex or withdrawing from combat; the movement cost would be the same. Thus, Bala and Albert can move around each other as they fight, seeking the best possible advantage depending on their surroundings. Remember, if either is stunned, they can be forced back, into a wall, a tight space or even off a cliff.
For game purposes, combatants may not enter an opponent's hex, as this space is considered defended. Grappling is an exception to this rule. Like the rules under stun lock, combatants may also be forced out of a hex through charging and overbearing. These are unusual experiences and must be specifically declared. Normally, if an unarmed defender enters a melee hex, that defender is considered engaged in melee; but a defender facing an unarmed opponent is not considered so engaged.