/* 

Every time the program encounters circleArea(argument),

it is replaced by (3.1415*(argument)*(argument)).

*/

#define circleArea(r) (3.1415*(r)*(r))

A macro is defined at the top of your program.

for eg: #define PI 3.14

Now whenever you write PI in your program 'PI' is replaced by 3.14

Actually this replacement is done by the preprocessor before the source code is compiled.

​