• It is defined as the ratio of a circle's circumference C to its diameter d is called pi.
  • It is denoted by symbol π.
  • But it turns out to be an "irrational number," meaning its exact value is inherently unknowable. Computer scientists have calculated billions of digits of pi, starting with 3.14159265358979323…, 
  • Pi is often shortened further to 3.14.
     Applications of Pi:
  • Science: Formulae from other branches of science also include π in some of their important formulae, including sciences such as statistics, fractals, thermodynamics, mechanics, cosmology, number theory, and electromagnetism.  
  • Trigonometry: It is also used to get the value of trigonometry function like sine, cosine, tangent...etc. It can measure circular velocity of things like truck wheel, motor shafts, engine parts, gears.
  • Geometrical designs: Calculation of surface area, volume of curved objects i.e- Cylinder, Cone, Sphere etc, like calculation of area & volume of machine parts requires Pi. Designing of bridges, highway curves requires Pi.   
Examples:
 Pi designed clock

Pi designed tires