3.1415926535897932384626433832795...
1/2 of a circle's circumference in radians (1 radius in radians.)
image courtesy of http://xkcd.com/
Note: to make scripts run faster, instead of useing the DEG_TO_RAD command to convert angles, use the following, PI = 180 degrees, PI_BY_TWO = 90 degrees, TWO_PI = 360 degrees, other useful angles PI/4 = 45 degrees, 3*PI = 270 degrees, PI/3 = 60 degrees, PI/6 = 30 degrees.
Constants | PI_BY_TWO | TWO_PI