I honestly can't believe this challenge does not already exist.
Write a function.
Your program must define some sort of callable function. This includes anything commonly known as a function, a lambda function, or a subroutine. All of these types of callables will be referred to as a "function" in this post.
Input to the function is optional and not required.
A return value from the function is also optional and not required but control must return to the calling program.
The function must be assigned to some sort of variable so that it is possible to be accessed at a later time. This includes indirect assignment (in most common languages where declaring a named function automatically adds the name into the current scope) and direct assignment (assigning an anonymous function to a variable directly).
The function does not need to be named.
The function must be created by you - you cannot just assign a default function from the language to a variable.
None of the standard loopholes, please.
This is code-golf, so lowest score in bytes wins.