noun
rule; custom; established practice
Refers to a settled rule, custom, or fixed way of doing things. Often implies something that has become standard or expected over time. The glosses 'tranquility'/'tranquillity' are rare and likely refer to a state of settled calm; this usage is uncommon in modern Japanese.
この村には古くからの定まりがある。
This village has long-established customs.
会議の定まりに従って進行する。
We proceed according to the meeting's rules.