Established, fixed, or predetermined. Often used in the phrase 'default/predetermined ~' to indicate a standard or prearranged setting.
既定の設定で問題がなければ、そのままお使いください。
If the default settings cause no problems, please use them as they are.
会議は既定の日時に行われます。
The meeting will be held at the predetermined date and time.
既定路線に沿ってプロジェクトを進める。
Proceed with the project along the established course.