pi

PI may refer to:

• Isoelectric point, the pH at which a particular molecule or surface carries no net electrical charge.
• Parental investment, in evolutionary biology
• Paternity Index, a value used to calculate probability of paternity
• Phosphatidylinositol, a class of lipids
• Pistillata, a gene that influences the development of flowers in the ABC model of flower development
• Plasticity index, a measure of the plasticity of a soil
• Polyimide, a polymer of imide monomers
• Ponderal index, a measure of leanness of a person (similar to body mass index)
• Principal investigator, the lead scientist or engineer for a particular project
• Propidium iodide, a chemical used as a DNA stain
• Protease inhibitor (pharmacology), class of drugs used to treat or prevent infection by viruses, including HIV and Hepatitis C
• Protease inhibitor (biology), molecules that inhibit the function of proteases
• Pulsatility index, a ratio of blood flow rates

The number is a mathematical constant, the ratio of a circle's circumference to its diameter, commonly approximated as 3.14159. It has been represented by the Greek letter "" since the mid-18th century, though it is also sometimes spelled out as "pi" .

Pi or is a mathematical constant equal to a circle's circumference divided by its diameter.

Pi (; Greek: , uppercase Π, lowercase π) is the sixteenth letter of the Greek alphabet, representing . In the system of Greek numerals it has a value of 80. It was derived from the Phoenician letter pe . Letters that arose from pi include Cyrillic Pe (П, п), Coptic pi , and Gothic pairthra .