Mutual knowledge is a fundamental concept about information in game theory

Game theory

Game theory is a mathematical method for analyzing calculated circumstances, such as in games, where a person’s success is based upon the choices of others...

 and logic

Logic

In philosophy, Logic is the formal systematic study of the principles of valid inference and correct reasoning. Logic is used in most intellectual activities, but is studied primarily in the disciplines of philosophy, mathematics, semantics, and computer science...

. An event

Event (probability theory)

In probability theory, an event is a set of outcomes to which a probability is assigned. Typically, when the sample space is finite, any subset of the sample space is an event...

 is mutual knowledge if all agents know that the event occurred . However, mutual knowledge by itself implies nothing about what agents know about other agents' knowledge: i.e. it is possible that an event is common knowledge but that each agent is unaware that the other agents know it has occurred. Common knowledge

Common knowledge

Common knowledge is knowledge that is known by everyone or nearly everyone, usually with reference to the community in which the term is used. Common knowledge need not concern one specific subject, e.g., science or history. Rather, common knowledge can be about a broad range of subjects, including...

is a related but stronger notion in that any event that is common knowledge is also mutual knowledge.