in terms of pi

Pi is just used as one of the factors that make up the number...

 

example:   Circumference of circle with 10 inch diameter    circ = pi x d  

                    the circumference would be expressed as  10 pi    (which means 10 x pi)

 

Basically ....you just leave 'pi' in the calculations and result rather than substituting in 3.14156 and multiplying it out.