#define name value
The #define directive assigns the given value to the specified name. All subsequent occurrences of the name are replaced by the value.
name
Specifies the name to be defined. This value is any combination of letters, digits, and punctuation.
value
Specifies any integer, character string, or line of text.
This example assigns values to the names “NONZERO” and “USERCLASS”:
#define NONZERO 1
#define USERCLASS "MyControlClass"
#ifdef, #ifndef, #undef