#pi#, which is roughly equal to 3.14159265... is the ratio of a circle's circumference to its diameter. No matter how big or small the circle is, if it's a perfect circle its circumference will always be #pi# times its diameter.

#pi# was not a concept that was "invented" or "proposed", but rather it was discovered. It is guessed that #pi# was discovered by Egyptians or Babylonians some 3,000-4,000 years ago, however most values of #pi# back then were only approximations such as #25/8#, or the most well-known approximation #22/7#. Neither of these approximations are good enough for use in any application of modern mathematics.

#pi^c# is simply #pi# raised to the power #c#, whatever #c# happens to be. So if #c=3# then #pi^c=pi^3=pi*pi*pi#.