define is a directive used in C and C++ to define a macro. It is followed by the name of the macro and the value that it should be replaced with whenever the macro is referenced in the code. The syntax is as follows:

#define MACRO_NAME value

For example, if you define a macro like this:

#define PI 3.14

Then every occurrence of "PI" in the code will be replaced with "3.14" during preprocessing.