"indicator function" or a "characteristic function" is a function defined on a set X that indicates membership of an element in a subset A of X, having the value 1 for all elements of A and the value 0 for all elements of X not in A. Here's more information on indicator function.
https://en.wikipedia.org/wiki/Indicator_function

"indicator function" or a "characteristic function" is a function defined on a set X that indicates membership of an element in a subset A of X, having the value 1 for all elements of A and the value 0 for all elements of X not in A. Here's more information on indicator function. https://en.wikipedia.org/wiki/Indicator_function