Functional

1. A functional is a real-valued function on a vector space, usually of functions.
2. Functional means relating to the way in which something works or operates, or relating to how useful it is.

A functional is a generalization of the concept of a function, which can have the meaning of a numerical function defined on some functional space, or a numerical function defined on an arbitrary linear space. An example of the most studied functionals is the class of linear functionals.

Sources:

YourDictionary https://www.yourdictionary.com/

Science News For Students https://www.sciencenewsforstudents.org/article/

The wonders of physics https://wonders.physics.wisc.edu/