An early name for calculus of variations. The term is also sometimes used in place of predicate calculus.
Functional Calculus
See also
Calculus of Variations, Functional Derivative, Predicate CalculusExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Functional Calculus." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/FunctionalCalculus.html