What is the prototype of a function?

Declaration of function is known as prototype of a function.

Prototype of a function means
(1) What is return type of function?
(2) What parameters are we passing?
(3) For example prototype of printf function is:

int printf(const char *, …);

I.e. its return type is int data type, its first parameter constant character pointer and second parameter is ellipsis i.e. variable number of arguments.