Declaration vs definition in C

Question

Consider the code:

int main(void)
{
    int a;
}

As far as I know, int a; is a definition, as it causes storage to be reserved

Answer 1

int a;

This is a definition There is a memory allocated for variable a

extern int a;

This is a declaration. Memory is not allocated because it is not defined.

Once a variable is defined you can use the address of it which is totally legal.