Pi

PI is the mathematical constant, commonly denoted π, which expresses the
ratio of the circumference of a circle to its diameter (or twice the
radius). It is useful in trigonometric calculations and functions (like
sine), and an intrinsic part of vector-coordinate
conversion (such as lengthdir_x,
lengthdir_y, and
point_direction), as well as
degree-radian conversion. PI is an irrational number, so most
applications will settle for an accurate estimate.

The constants pi and M_PI are both
defined in EDL, so you can use them in your code.
Game Maker only defines pi.

• In GM, pi = 3.141592653589793280 (Accurate to 15 places, precise to
  16)
• In ENIGMA, pi = M_PI =
  3.1415926535897931159979634685441851615905761718750000000000000000
  (Accurate to 15 places and precise to 48. This is the most accurate
  and precise representation of PI that a double can hold)
• In Math, pi = 3.14159265358979323846... (Accurate and precise to 20
  places. Attempting to use or memorize any more than 20 places is
  quite difficult.)

Tau

Tau (or τ) is commonly suggested as an alternative to pi, defined as 2
pi (the ratio of the circumference of a circle to its radius), as it is
more common to use the radius of a circle in trigonometry rather than
its diameter. Despite this, most applications do not define this as a
constant (including GM and EDL)