Thai: ข่า (khaa)
Galangal is the khaa in Tom Khaa. Also used in Tom Yum and curry pastes. You can not substitute ginger for galangal, or any other root. You can usually find it frozen in South East Asian groceries. Sometimes Chinese groceries carry it too — you may have to ask. Galangal is recognizable by the long pink stalks and pink tips on the root.
Storage: Keep in the refrigerator, or store sealed in the freezer.