noun
A mathematical term for a function whose input is a function and whose output is a scalar. Used in fields such as functional analysis and calculus of variations.
汎関数は、関数を引数にとり実数値を返す写像である。
A functional is a mapping that takes a function as an argument and returns a real number.
関数 is a standard function mapping numbers to numbers, while 汎関数 maps functions to numbers.
Compound of 汎 (han, 'broad, general') and 関数 (kansū, 'function'), likely a calque from English 'functional'.