noun which may take the genitive case particle 'no', noun
unopened; sealed
Describes an item that has not been opened and still has its original seal intact, such as a package, envelope, or product.
An unopened envelope was on the desk.
Please return this product unopened.
未開封 means 'unopened' (the packaging hasn't been broken), whereas 未使用 means 'unused' and can apply even to opened items that were never actually used.
Compound of 未 (mi, 'not yet') + 開封 (kaifū, 'opening a seal'). The reading is straightforward on'yomi.