during; for the duration of
Used to indicate that something happens or is valid throughout a specified period. Often follows a time expression (e.g., 夏休み期間中, キャンペーン期間中).
During the summer vacation period, the library's opening hours will be shortened.
キャンペーン期間中にご注文いただくと、送料が無料になります。
If you order during the campaign period, shipping is free.
Many items are discounted during the sale period.
Shipping is free for the duration of the campaign.
This discount is valid only during the summer vacation period.
中 attached directly to a time noun (e.g. 今月中) also means 'during', but 期間中 explicitly names the 'period' itself, making it clearer when the period is the topic of discussion.
Compound of 期間 (period) + 中 (during). A straightforward Sino-Japanese formation.