Unique
[16] Buzzbox 525.Some items in the game are flagged as unique. You can only have one copy of a unique item in all of your inventory, including what you have in the bank. If presented with another opportunity to acquire another of the same item then the character will not be able to loot the item.
It is possible to have another player mail a second identical item to a character that has a unique item, but they will be unable to retrieve it from their mail until disposing of the previous unique item. The mail "trick" simply gives another person control of the unique.
To remove a unique item from the mailbox, while already possessing one, a player must sell or delete the item in their possession then collect the second unique item from the mailbox. They then dispose of it appropriately (such as mailing to another character) and use the "buyback" feature to return the original item to their inventory.
A handful of items are "Unique (X)", meaning a player can only have X copies of that item between their inventory and bank. For example, if a player has five of an item with the "Unique (5)" property, they cannot loot or otherwise acquire any more of them.
Unique-equipped
If an item is labeled as unique-equipped, it means that a player can only have one of those items equipped at any one time. A player can have as many as desired in their inventory, but only one can be equipped to a slot at a time. Some particular items (namely Jewelcrafting-specific gems) permit a specific number (greater than one) to be active simultaneously.
This restriction is mostly to prevent the overpowering of characters with powerful items, while still allowing them to craft and sell these items to others even if they already have one. It is also used on rings and trinkets, especially epic ones, to prevent equipping two of them while still allowing a player to loot more than one, presumably to sell or disenchant.
Patch changes
Patch 2.3.0 (2007-11-13): Dropped items that were rings/trinkets/one handed weapons that were previously unique are now unique-equipped. That means that you can have more than one of those items, but you can only have a single one of them equipped.