#define is a useful C component that allows the programmer to give a name to a constant value before the program is compiled. Defined constants in arduino don’t take up any program memory space on the chip. The compiler will replace references to these constants with the defined value at compile time.
This can have some unwanted side effects though, if for example, a constant name that had been #defined is included in some other constant or variable name. In that case the text would be replaced by the #defined number (or text).
In general, the const keyword is preferred for defining constants and should be used instead of #define.
#define constantName value
Note that the # is necessary.