Perfect information

Perfect information has different definitions depending on the field.
In economics, the term perfect information refers to an element of competition in a market which is perfect and idealized. This is commonly known as perfect competition.
In the mathematical branch of game theory, a perfect information game uses perfect information if all the actions taken in the game are known, and the state of the game's materials is available to all players. Perfect information is a type of common knowledge. Perfect information relates to the materials of the game, and to the rules of the game.
It follows that imperfect information is when elements of the game (materials or rules) are not equally available to all participants. 
Perfect information games do not require that each player has a working memory of past positions but that merely they have had the opportunity to view such past actions.
Examples.
In a game of chess or go, the pieces and the rules are known, but the implications of the position are not. This identifies chess as a perfect information game. In competitive card games like bridge and poker, the cards held by opponents are not known, or only partly known. They are not perfect information games.