Unique

From WoWWiki

Some items in the game are flagged as Unique. You can only have one unique item in all of your inventory including what you have in the bank at a time. If you have an opportunity to acquire another of the same item, you will be unable to. For example, if a mob drops the same item, you won't be able to loot the item, and if the mob is skinnable, this will prevent you from skinning the mob.

This can be particularly troublesome with unique bag items, since you want to have over a dozen bags, total. If you are upgrading a bag and have a choice of upgrading a unique or a non-unique with the same number of slots, replacing the unique bag gives you one less potential problem.

It is possible to have someone mail you a second identical item, but you will be unable to retrieve it from your mail until you get rid of your unique, and they will be unable to collect another identical item. The mail "trick" simply gives another person control of the unique so you can't use it.

To remove a unique item from your mailbox if you already possess one in your inventory, sell the item in your possession to a vendor, remove the second unique item from the mailbox, dispose of it appropriately (such as mailing to another character), then use the buyback feature to return the original item to your inventory.

A handful of items are "Unique (X)", where you can only have X copies of that item between your inventory and bank. For example, if you have five of an item with the "Unique (5)" property, you could not loot or otherwise acquire any more of them.

Some items are Unique-Equipped, which is not nearly as restrictive.

Very few items are "true unqiues"; items that only one or two people in the entire server may possess, such as the [Black Qiraji Resonating Crystal]- the only legendary mount in the game.