na-adjective
Describes something that follows a fixed pattern, schedule, or set of rules. Often used for habits, schedules, or orderly processes.
彼は規則的にジムに通っている。
He goes to the gym regularly.
規則的な生活を送ることが健康に良い。
Leading a regular lifestyle is good for your health.
定期的 emphasizes fixed intervals (periodic), while 規則的 emphasizes adherence to a pattern or rules (systematic).
Formed from 規則 (rule, regulation) + 的 (-like, -ic), a common pattern for creating adjectival nouns.