A form of Tabletop/Video Game combat where players and their units act in turns. Combat time is split into chunks (turns), during which individual units can act in more or less fixed order. While a player contemplates their next action, Time Stands Still for everyone on the battlefield.
Turn-based combat is one of the most glaring Acceptable Breaks from Reality: while utterly unrealistic, its major appeal lies in the ability to abstract the chaotic mess that is Real Life combat with a few concise gameplay rules. Furthermore, its implementations tend to go easier on video game hardware than real-time combat, and it also allows for more gameplay complexity, since the players have all the time they need to review all options and select the best course of action.
Turn-based combat will often but not necessarily feature some of the Common Tactical Gameplay Elements. Compare Real Time with Pause, which is sometimes used to hide a turn-based move and attack resolution behind seemingly real-time gameplay. See also Sliding Scale of Turn Realism.
All War Gaming, Tabletop RPG, Turn-Based Tactics, and Turn-Based Strategy games feature turn-based combat by definition, making it an Omnipresent Trope in those media/genres, so please put those examples directly on the respective genre page.
When do you give commands?
Start of the Turn. At the start of each turn, each player gives the orders to every unit under their command. These orders are then carried out either simultaneously (so-called "Simultaneous Turn Resolution") or in order of initiative (see below).
Before the Unit Acts. The player gives orders to each unit individually and they are carried out immediately.
When is it a unit's turn to act?
Initiative Queue: Each unit, regardless of allegiance, is assigned an initiative score (partly randomized, partly dependent on its stats) at the start of combat: the one with the highest initiative gets to act first (as long as it is not killed), then the second highest, and so on. After the lowest initiative has acted, the next turn begins. The highest initiative may act first again, or the initiative may be recalculated every turn. A Visual Initiative Queue may be displayed.
One Side, One Turn: Each player (whether human or AI) gets to move each of their units individually in any order on their turn, while the enemies stand still. Afterwards, the other player gets to move all of their units in the same manner, and so on.
One Side, One Move: Each player gets to move with a single unit on their turn, after which the other player moves one of theirs, and so on. Chess works this way.
Action Types: On its turn, each unit can perform one action of a specific type, e.g. a "move action" and an "attack action", which allows it to move in position and attack the enemy, or attack and move away. Some action types can be traded for others (e.g. the attack for another move—but not vice versa), while some things don't require using up an action at all.
Action Points: Each unit gets a pool of "action points" each turn, which represent its quickness. Taking an action consumes a varying number of points from this pool until it is empty and the unit can no longer act on this turn. Leftover action points are not carried over to the next turn.
How far can you move on your turn?
Field Grid: The battle area is overlaid with a grid of square- or hexagon-shaped fields, and a unit can move up to a certain number of fields on its turn. A single field may only be occupied either by an inanimate obstacle, or by a single living unit at one time (although allies may be able to pass through each other's fields).
Continuous Terrain: There is no grid, and the unit can move to any point on the battlefield where the collision detection algorithm allows it to, with the cost of the move calculated by the appropriate pathfinding algorithm.
Can you act out of turn?
Delayed Action. A unit can choose not to attack on its turn, but instead to delay the attack until the enemy does something specific (e.g. comes into shooting range), effectively getting to act on the enemy's turn.
Attack of Opportunity. Under certain conditions, a unit can make a free attack out of turn, triggered by an enemy's action, such as firing a ranged weapon at close range, or attempting to disengage from melee.
The Russian RPG Golden Land switches between real-time exploration and turn-based combat similarly to the Fallout series.
Ultima IV featured a simplistic turn-based combat wherein the player could perform a single action (move one square, attack in one of the directions, cast a spell, or skip turn) with each party member in the order they were recruited, after which every enemy got to act in the same way, and so on.
Divinity: Original Sin features tactical turn-based combat with initiative, action points, and free movement (no grid).
The Banner Saga is centered around tactical action point-based combat on a squared movement grid.
Final Fantasy I, II, and III used turn-based combat before the introduction of the Active Time Battle system in later titles. However, it is also used in the relatively recent Final Fantasy: The 4 Heroes of Light (which is also a spin-off of the core Final Fantasy series) and its even more recent Spiritual SequelBravely Default (which also takes some elements from the FF series), mostly because these two are throwbacks to classic Final Fantasy (4 Heroes of Light, to the NES era FF and Bravely Default, to the SNES era FF sans ATB).
The Uncharted Waters series switches to turn-based combat mode during naval battles. In Uncharted Waters: New Horizons, sword duels between fleet captains are also fought like this, with each combatant attacking and defending in turns.
The main Shin Megami Tensei series tends to a be variation of One Side, One Turn where your party members act in order (based on the position in your party) until you run out of actions, and then the enemy goes in the same fashion. While they fit into the basic One Attack formula, the twist is that you can have more actions per turn than you have party members, and can gain or lose them based on what you do during the turn, generally gaining extra actions for critical hits and targeting weakness, while losing them for missing or targeting resistances. You don't have the freedom to choose who uses which actions, so the extra actions go to people further up on your party list, making formation very important. There are no out-of-turn actions except basic reprisal attacks on some characters.
In Golden Sun, actions are made at the beginning of every round with each party member going after the other. In some cases a party member can act twice.
The Dragon Quest series uses turn-based combat and, from DQ4 onwards, included a limited party member AI (it is possible to order them to automatically heal allies, enemies, etc.), which can speed up the combat considerably but is best to turn it off for boss battles.
In Epic Battle Fantasy, orders are not given at the beginning of a round but individually, making it easier to react to battle events (particularly useful against a boss whose resistance changes after every attack).
The Disgaea series uses a pure One Side, One Turn system, with more freedom than most. Because of the emphasis on a combo bonus for multiple attacks on the same target, you're encouraged to move all your characters into position before making them attack, and in fact there's an "execute" command you need to use in order to make anything actually happen, upon which the actions occur in the order you selected them. There's also some meta-fiddliness you can do involving placing characters into position to support attackers, executing, and then canceling the movement. Since the support character never actually acted, they can then move again to support other attackers repeatedly until finally acting themselves. This doesn't make much logical sense in terms of movement rates, but it's a very "gamey" series that encourages clever abuses of game mechanics like that. As inferred, it uses One Move And One Attack and a Field Grid. Other games not in the main series have experimented with Continuous Terrain but have always felt awkward. Sometimes they also use a non-turn based initiative system like FFX.
Soul Nomad & the World Eaters is similar to Ogre Battle in that you construct parties that, for tactical purposes, acts as a single unit. Whenever this unit comes into contact with an enemy unit, themselves a full party, on the tactical map, it zooms in and has a little mini-battle plays out where each member of the party goes through their predetermined and limited action set (i.e. no more micromanaged player choices happen at that point). After that, the mini-battle ends regardless of whether or not any party-unit was fully destroyed, and the tactics game begins again. There's often a lot of complicated fiddling you can do with formations or side attacks or whatever. Ultimately though, it's no different from the One Move and One Attack thing combined with Field Grid, except each "unit" and each "attack" is a slightly more complicated array of mini-units with their own mini-actions.
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