Why are card games described as games of imperfect information?

Card games are described as games of imperfect information because they utilize playing cards that are individually identifiable from only one side. Consequently, each player possesses knowledge only of the cards they hold, while the cards held by opponents remain hidden.12

This classification distinguishes them from games of perfect information, such as chess, where the entire state of the game is visible to all participants at all times. In most card games, the initial distribution of cards is randomized through shuffling, meaning the "imperfection" of information relates to the unknown distribution of cards at the start of play. As the game progresses, players use observation—such as tracking bids, discards, and trick play—to deduce this distribution, effectively transforming the experience into one of increasing information.12

It is important to note that this label is not universal; for instance, certain varieties of solitaire are considered games of perfect information because all cards are dealt faceup.1