A power system is the set of rules governing abilities in a fictional world: where power comes from, its categories, costs, limits, and how users grow. Nen, chakra, Devil Fruits, and cursed energy are examples of named power systems.
A power system is a promise about fairness. If abilities follow rules, the audience can reason about fights before they happen, feel a clever use as clever, and feel a violation as a cheat. Series advertise their systems early and build mysteries inside them: the undiscovered application, the hidden category, the exception that turns out to be a deeper rule.
Systems grow like languages. A few base mechanics combine into techniques, techniques specialize by user, and mastery becomes expression: two characters with the same power fight nothing alike. Databooks and guides formalize the grammar, and fans memorize type charts and category lists with a diligence school never got from them.
The system is also where canon discipline bites hardest. Fan wikis routinely promote speculation into system facts, and matchups argued from invented rules are fandom's favourite unwinnable argument. This site quizzes only the system as the work itself states it, and the canon sheet records which source, chapter, or guide a rule comes from. The most respected systems are those an author refuses to break: when the rules inconvenience the plot, the plot bends, and the audience's trust compounds like interest.
In use"A world where abilities fall into six categories with known strengths and costs has a power system, and fans will chart it."