Active Time Battle
In a simple queue-based combat engine, the ‘empty time’ between units is simply skipped as if it didn’t matter. In an Active Time Battle game, however, that time is experienced by the player, usually visualized as some kind of energy bar that fills up for each of the player’s units. (Enemy units don’t usually have the bar.) Once the bar is filled, the unit can be issued a command. This model, initailly made popular with Final Fantasy IV, combines some real-time aspects on top of what is fundamentally a queue-based system - if the player isn’t quick enough, enemy units will get to act before their own and their own units will essentialy “waste” time doing nothing.