c - Difference between "defining" and "declaring"

Declaring is telling the compiler that there's a variable out there that looks like this.

Defining is telling the compiler that this is a variable.

One refers to the existence of a thing, the other is the thing.

In your example, the scope is what makes the difference. Declarations are made in a file scope, but in a block scope it is not possible to declare anything; therefore, the second example is a definition; because, there is nothing left to do with the int x;.

That makes the first example (in the file scope) a declaration that some int x; exists. To covert it from a declaration, you need to specify that a value is assigned to it, forcing the memory allocation. Like so: int x = 0;

C and C++ are very scope sensitive when it is analyzing constructs.