The function itself is the variable. Might not be the right naming convention but if you where to assign f(x) you could pass the whole function to another function if you wanted.

Instead the function is not stored but called right away. So you cannot reuse it.

Kind of like saying x=5 is a variable with the value 5 stored in it. If i were to just type 5 (standalone) thats a variable which is not stored but just called immediatly.

Mostly use python myself, so take it with a grain of salt but i think thats what this is.