Binds to Account (BtA) is a new form of item binding that was introduced in Patch 3.0.2. Heirlooms will be BtA, as well as the Collector's Edition companions.
Also see
- Soulbound
- Bind on Pickup (BoP) or Bind on Acquire (BoA)
- Bind on Equip (BoE)
- Bind on Use (BoU)