Difference between functional and function. I have come across the term 'functional'. How is a 'functional' different from a 'function'? The exact term I came across was 'statistical functional.'
In terms of the background, can you please focus on the first few lines followed by the four examples in http://sites.stat.psu.edu/~dhunter/asymp/fall2002/lectures/ln14.pdf ?
The definition is seeming to be quite generalized given the examples. Is there a crux of the definition that is not too general nor too restricted?

Typically, a functional is just a function of a set into a field $F$. The ones I know best are "linear functionals," where a functional is a linear function of an $F$ vector space into $F$.
I'm afraid I don't know about the "statistical" reference, I just know the above is true in functional analysis and some applied mathematics that I've read.

Added:
Ah, I also forgot that sometimes people call "functions of functions" functionals.
