Key Items are items in 异度神剑, 异度神剑 2 and Torna ~ The Golden Country.
异度神剑

The icon for Key Items in 异度神剑
Key Items are used specifically for quests. Many of them can be collected regardless of whether the quest has been received or not. They appear as small glowing red balls, similar to Collectables aside from the colour. If a quest to find a Key Item has been accepted, its location will be marked on the mini-map by a red exclamation mark. Key Items cannot be sold, and have their own section in the inventory.
异度神剑 2

The icon for Key Items in 异度神剑 2
Key Items are usually required or rewarded during quests.
Some key items can be used once for various effects while 其它, like Store Deeds have a passive effect upon being obtained.
Torna ~ The Golden Country

A Leggin Parisax with the Leaden Slate Piece
Key Items are usually required or rewarded during quests.
Slate Pieces
Slate Pieces are sixteen key items in total, which must be collected from all five regions in the Kingdom of Torna in order to unlock the Cavern of the Seal and thus access the Tasteless Altar and the 诺彭 Halfsage.