The period during which something (a ticket, document, coupon) is valid and effective.
クーポンの有効期間は今月末までです。
The coupon is valid until the end of this month.
パスポートの有効期間を出発前に必ず確認してください。
Be sure to check your passport's validity period before departure.
チケットの有効期間内に公演にご来場ください。
Please attend the performance within the ticket's validity period.