What is the difference between a defining instance and an external declaration?

A defining instance is a declaration of a variable or function that actually defines and allocates space for that variable or function. In the case of a variable, the defining instance may also supply an initial value, using an initializer in the declaration. In the case of a function, the defining instance supplies the body of the function.

An external declaration is a declaration which mentions the name and type of a variable or function which is defined elsewhere. An external declaration does not allocate space; it cannot supply the initial value of a variable; it does not need to supply the size of an array; it does not supply the body of a function. (In the case of functions, however, an external declaration may include argument type information; in this case it is an external prototype declaration.)