Define a constant (e.g. $\pi$ ):
#define PI 3.14159
Can also define operations, such as (called syntactic sugar):
#define EQ ==
Or also used sometimes to define new operations:
#define SQ(x) (x*x)
Not recommended, as it may cause unintentional errors.
Define a new type, e.g. turn enum day into day:
typedef enum day day