Blank expression

A blank expression is a facial expression characterized by neutral positioning of the facial features, implying a lack of strong emotion. It may be caused by a lack of emotion, depression, boredom or slight confusion, such as when someone refers to something which the listener does not understand. A deliberately-induced blank expression meant to conceal one's emotions is also known as a poker face, referring to the common practice of maintaining one's composure when playing the card game poker.

The term poker face was used outside the game of poker by American sportswriters in the 1920s to describe a competitor who appeared unaffected by stressful situations (an important skill when playing poker for money, to avoid giving your opponent any tells about your hand). It is often used by mental health professionals to describe certain patient behaviors, and by marketers and salespeople during business negotiations.

...
Wikipedia