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An “action economy” is a gamist term that ties into the mechanic side of RPGs as well as most board games and even some card games. The idea being that you can only do so much in a single turn, so you want to maximize each turn by taking the greatest number of actions.

This was most prevalent in 3rd Edition and 4th Edition that had multiple types of actions, making it possible to find spells or powers usable in multiple different ways. There were also often ways of adding even more options via spells, animal companions, or magic items. The more things you could do in a single turn the more powerful your character.