Heirloom is a characteristic of an item that restricts who it can be traded to. An item flagged as heirloom can not be traded to other players unless they were in the group when it was looted, and then it can only be traded to those specific players within 48 earth hours of looting the item.
However, an item flagged as heirloom can be placed in your personal shared bank slots and traded between characters on the same account at any time.