In a deductive system with a binary connective , describes the binary connective satisfying the following scheme for all , , and :
iff
In classical logic, for example, material implication is the residuum (or residual) of conjunction. Likewise, in relevant logic, relevant implication serves as the residuum for the binary connective of fusion. The notion is especially important in the case of fuzzy logic, in which distinct species of fuzzy implication are defined as the residua of distinct triangular norms.