I am using calc
often (from the terminal) and I was wondering if there is a PI constant in there, predefined, somewhere?



If you are talking about calc from the apcalc package the constant Pi is defined like this:
note, the parentheses are used even though the function does not take any arguments. not too clear for the GUI desktop calculator,gcalctool, either, which is: Ctrl+P 


you also can choose kcalc there is a scientific  mode in kcalc there again you can choose mathematical constants e.g. PI, Euler, golden ratio ... if you want to tickle more out of PI in own written program you can use then the library of http://gmplib.org/ this library you can use when compiling with gcc resp. g++ have fun ! 

