the #define method of declaring a constant enables you to declare a
constant in one place and use it throughout your program. This helps
make your programs more maintainable, because you need to maintain
only the #define statement and not several instances of individual
constants throughout your program.
For instance, if your program used the value of pi (approximately
3.14159) several times, you might want to declare a constant for pi
#define PI 3.14159
Using the #define method of declaring a constant is probably the
most familiar way of declaring constants to traditional C
programmers. Besides being the most common method of declaring
constants, it also takes up the least memory.
Constants defined in this manner are simply placed directly into
your source code, with no variable space allocated in memory.
Unfortunately, this is one reason why most debuggers cannot inspect
constants created using the #define method.