In epistemic logics with knowledge operators corresponding to multiple agents, any formula that is known by every agent. More formally, when there exist knowledge operators ${\text{K}}_i$ indexed by a set of agents $I$, $\phi$ is mutual knowledge if:

• • for all $i\in I$, ${\text{K}}_i\phi$

Note that mutual knowledge of $\phi$ is weaker than the related notion of common knowledge. That $\phi$ is mutual knowledge between agents $0$ and $1$ does not entail that ${\text{K}}_0{\text{K}}_1\phi$ holds, i.e., that any agent knows that $\phi$ is mutual knowledge.