A formal system which extends first-order logic by allowing quantification over relations between members of the universe.

(logic) A formal deductive system which extends first-order logic by the ability to quantify over unary predicates over individual members of the universe of discourse.

Second-order cybernetics, also known as the cybernetics of cybernetics, is the recursive application of cybernetics to itself and the reflexive practice of cybernetics according to such a critique.

A second-order cone program is a convex optimization problem of the form

In mathematical logic, second-order arithmetic is a collection of axiomatic systems that formalize the natural numbers and their subsets.