1A conjunction meaning 'but, however, provided that' — used to attach an exception, condition, or qualification to what was just stated. More formal than しかし and common in written instructions, notices, and contracts.
入場は無料です。ただし、事前登録が必要です。
Admission is free. However, prior registration is required.
商品は返品できます。ただし、未使用のものに限ります。
Items can be returned, provided that they are unused.
明日は休みます。ただし、緊急の連絡には対応します。
I'll be off tomorrow. However, I will respond to urgent messages.
この契約は自動更新される。ただし、解約は1か月前までに通知するものとする。
This contract is renewed automatically, provided that notice of cancellation is given at least one month in advance.