II.16: What is the difference between declaring a variable and

Answer:
Declaring a variable means describing its type to the compiler but not allocating any space for it. Defining a variable means declaring it and also allocating space to hold the variable. You can also initialize a variable at the time it is defined. Here is a declaration of a variable and a structure, and two variable definitions, one with initialization:

extern int decl1; /* this is a declaration */
struct decl2 {
int member;
}; /* this just declares the type--no variable mentioned */
int def1 = 8; /* this is a definition */
int def2; /* this is a definition */

To put it another way, a declaration says to the compiler, “Somewhere in my program will be a variable with this name, and this is what type it is.” A definition says, “Right here is this variable with this name and this type.”

NOTE
One way to remember what each term means is to remember that the Declaration of Independence didn’t actually make the United States independent (the Revolutionary War did that); it just stated that it was independent.
A variable can be declared many times, but it must be defined exactly once. For this reason, definitions do not belong in header files, where they might get #included into more than one place in your program.

Reference:
II.17: Can static variables be declared in a header file?