Bind on Account

Bind on Account, or BoA designates an item that "binds" to the account of the character who first picks it up. These types of items could be passed between different characters on the same account (presumably by using the shared stash), but could not be traded to another other characters.

It is not confirmed that there will be BoA items in Diablo 3, but the D3 Team has talked about using BoA as a way to keep the end game economy functional. The theory is that when top level items can be traded, eventually every character has the best of everything, since characters who indulge in huge amounts of item finding (or illegal duping, in the case of Diablo 2) will score enough of that item to distribute them widely.

Compare BoA to BoE -- Bind on Equip which is also known as "soul binding." These types of items bind to the character who first equips them; they can be traded, perhaps only within the same account, but once any character actually uses the item, it is bound to that character permanently. (Characters can dispose of such items for space considerations, but they can not give or trade them to other characters.)