I find it interesting how economics and game theory have so much in common. Here are definitions of two terms (courtesy of Wikipedia) that have identical meanings in both fields.

Fragment of Perfect Information definition (see entire definition here):

Perfect information is a term used in economics and game theory to describe a state of complete knowledge about the actions of other players that is instantaneously updated as new information arises.

Chess is the canonical example of a game with perfect information, in contrast to, for example, the prisoner’s dilemma.

In microeconomics, a state of perfect information is required for perfect competition. That is, assuming that all agents are rational and have perfect information, they will choose the best products, and the market will reward those who make the best products with higher sales. Perfect information would practically mean that all consumers know all things, about all products, at all times, and therefore always make the best decision regarding purchase. In competitive markets, unlike game-theoretic models, perfect competition does not require that agents have complete knowledge about the actions of others; all relevant information is reflected in prices.

Fragment of Complete Information definition (see entire definition here):

Complete information is a term used in economics and game theory to describe an economic situation or game in which knowledge about other market participants or players is available to all participants. Every player knows the payoffs and strategies available to other players.

Complete information is one of the theoretical pre-conditions of an efficient perfectly competitive market. In a sense it is a requirement of the assumption also made in economic theory that market participants act rationally. If a game is not of complete information, then the individual players would not be able to predict the effect their actions would have on the others players (even if the actor presumed other players would act rationally).

Although similar, complete and perfect information are not identical. Complete information refers to a state of knowledge about the structure of the game, while not necessarily having knowledge inside the game. So for example, one may have complete information in the context of a Prisoner’s Dilemma, but nonetheless this is a game of imperfect information since one does not know the action of the other player. Despite this distinction, it is useful to remember that any game of incomplete information can be transformed, terminology-wise, into a game of imperfect information. One simply includes nature as a player in the game and conditions payoffs on nature’s unknown moves.