3.14159... (π or pi) is the ratio of a circle's circumference to its diameter.
π is the area of a unit circle. It is also the ratio of the circumference of any convex curve of constant diameter to its diameter (Barbier's theorem).
π is an irrational number and a transcendental number.
22/7 is the best approximation to π, using the ratio of two numbers less than 100.