Kra-chai (Fingerroot กระชาย)

You can read about what fingerroot (Boesenbergia rotunda) is here.

If you can find fresh kra-chai, by all means, use it. If not, I highly recommend kra-chai slivers in brine as shown here. I've found it to be more potent than frozen, vacuum-packed kra-chai (which can come whole or slivered).