It is a bad practice to use ‘magic numbers’ or sequence of characters in a program, as it is difficult for a new person to understand the significance of the magic number in the program. Magic numbers are constants like 249,10 etc. The best way to replace those is a symbolic name or symbolic constant. A #define line defines it
#define name replacement list
The further use of the name will be replaced by the replacement text. Thereafter, in place of the magic number we can use the replacement text. The replacement list is not limited to numbers.
#define X sorry
Symbolic constants are conventionally written in upper case so that it could be distinguished from lower case variable names.Another important thing is there is no semi-colon at the end of #define line.