A poker face was a neutral facial expression employed to disguise one's feelings on a matter. It was so named because of its use in the card game of poker.
In 1893, Frederick La Rouque described Joe Falling Hawk's neutral expression during a card game as "a poker face carved in marble." The seaman folded rather than risk losing his Money. (TNG: "Time's Arrow")
In 2365, during one of USS Enterprise-D's regular poker games, Data questioned whether William T. Riker was putting on a poker face. (TNG: "The Measure Of A Man")
In 2366, Captain Jean-Luc Picard instructed Riker to put on his best poker face in order to fool Tomalak. (TNG: "The Enemy")
In 2367, during another poker game, Riker complimented Data's poker face, saying it was the best he'd ever seen. (TNG: "Legacy")
In 2375, impressed with Sarina Douglas's sharp insights into the character of several Deep Space 9 officers, Julian Bashir told her he'd have to start assuming a poker face around her. She assured him that it was already too late for that, as she had already formed an impression of him. (DS9: "Chrysalis")