poker face

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A face lacking any interpretable expression, as that of an expert poker player.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun poker An impassive facial expression cultivated to prevent other players from determining whether one's actions in the game are the result of a quality hand, or of bluffing.
• noun Any similar expression used to prevent giving away one's motives, feelings, or situation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a face without any interpretable expression (as that of a good poker player)