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'''Pi''' is the sixteenth letter of the [[Greek alphabet]] and is used in its lower case form (<big><math>\pi</math></big>) to represent the mathematical constant of the same name which is defined as ''the ratio of the circumference of a circle to its diameter''.  It is an important number and appears in many [[mathematical]] and [[physics|physical]] formulae.   

'''Pi''' (<big><math>\pi</math></big>) is the ratio of the circumference of a circle to its diameter: approximately 3.14159 (but mathematicians have struggled for centuries to provide an exact value). Using the convention that Greek letters in mathematics have conventional meaning, the ratio takes its name from the sixteenth letter of the [[Greek alphabet]].  It is an important number and appears in many [[mathematical]] and [[physics|physical]] formulae.   

The value of <big><math>\pi</math></big> is an [[irrational number]]; which means that it cannot be fully expressed as a fraction or a decimal (regardless of the number of digits used).  

The value of <big><math>\pi</math></big> is an [[irrational number]]; which means that it cannot be fully expressed as a fraction or a decimal (regardless of the number of digits used).