#define creates an entity for substitution by the macro pre-processor, which is quite different from a constant because depending on what you define it will or will not be treated as a constant. The contents of a #define can be arbitrarily complex, the classic example is like this:
#define SQR(x) (x)*(x)
Then later if used:
SQR(2+3*4)
That would be turned into:
(2+3*4)*(2+3*4)
