A turn is a period, in which certain actions may be taken by a Side.
Acting on Turn
Each day is divided up into a series of Turns, and each Side gets a single Turn per day. There is a natural turn order, resulting in a cyclic turn order. (Eg. Side A will always move before Side B, who will move before Side C, then night will pass and Side A moves again.) Note that alliances affect turn order.
During its Turn, a Side may:
- Heal (all units taking Turn heal prior to beginning Turn) 
- Move Units (up to the unit's Move value)
- Cast Spells
- Attack units
- Use Natural Abilities
Acting on Another Side's Turn
- Defend itself from attack
- Attack enemies only when enemies are in its hex. 
- Cast spells only when enemies are in its hex.  (Some spells can only be cast on own turn.)
- Automatically attack when in contact with non-allied units, unless a leader is present.
- Use Natural Abilities. 
If multiple Sides ally, they take turns simultaneously, on the Turn of the last ally. For instance, using the example above, if Side A allies with Side C, then Side B will take Turn before Sides A and C, who take Turn simultaneously. If Sides A and B ally, they will both still take Turn before Side C, but they will do so simultaneously instead of consecutively.
It appears that the length of day is not limited, and so neither is the length of a turn. This makes it useless to hold your end of turn for longer than necessary, because the sun won't set until everyone has finished their Turn.
Turnamancy may affect the Turn in some way.