Bind to Account
Bind to Account means that an item can only be transferred between characters on the same account. Items that are Bound to Account cannot be sent through the in-game mail, traded to another player, or put on the Auction House. Currently, the only way to transfer a Bind to Account item is through the Shared Storage, although barter items can also be shared via the Premium Wallet.
Rarely you will see the acronym BtAoA meaning "Bind To Account on Acquire" which in practise is 100% equivalent with "Bind to Account".