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.
This village has long-established customs.
We proceed according to the meeting's rules.
usual; stereotyped; predictable
Derived from the verb 定まる (さだまる, 'to become settled, fixed, or decided'), with the continuative form used as a noun. The kanji 定 means 'determine, fix, establish'.