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Phi (Φ, φ, or ϕ) is the 21st letter of the Greek alphabet.

Phi or PHI may also refer to:

Science and technology

Mathematics

• Golden ratio (φ)
• Phi coefficient, a measure of association for two binary variables introduced by Karl Pearson
• Euler's totient function or phi function
• Cyclotomic polynomial (${\displaystyle \Phi _{n}(x)}$)
• Integrated information theory (IIT) the symbol of which is φ is a mathematical theory of consciousness developed under the lead of the neuroscientist Giulio Tononi
• Standard normal distribution, ${\displaystyle \Phi (x)}$ notating its cumulative distribution function and ${\displaystyle \phi (x)}$ its probability density function

Physics, chemistry and biology

• Phi meson, in particle physics
• Magnetic flux (Φ)
• Peptide PHI (Peptide histidine isoleucine)
• 6-phospho-3-hexuloisomerase, an enzyme
• Phenyl group (Φ), a functional group in organic chemistry
• Pre-harvest interval
• pH(I), the isoelectric point

Computing

• Xeon Phi, an Intel MIC microprocessor
• Φ (Phi) function, in static single-assignment form compiler design

Medicine

• Permanent health insurance, against becoming disabled
• Protected health information, in US law

Other science

• Phi phenomenon, in visual perception
• Krumbein phi scale, for the size of a particle or sediment

Arts and entertainment

• Phi (KinKi Kids album) (2007)
• Phi (Truckfighters album) (2007)
• Sailor Phi, a villain in the Sailor Moon manga
• Phi, a character in the visual novels Zero Escape: Virtue's Last Reward and Zero Escape: Zero Time Dilemma.
• Phi: A Voyage from the Brain to the Soul, a book by Giulio Tononi (2012)
• Phi, a character from Beyblade Burst Turbo, a TV show written by Hiro Morita

Organizations

• Packard Humanities Institute
• Paraprofessional Healthcare Institute, a nonprofit organization based in New York City, US
• Pepco Holdings Inc.
• Petroleum Helicopters International, an American commercial helicopter operator
• Philadelphia’s major professional sports teams
  • Philadelphia Eagles of the National Football League
  • Philadelphia 76ers of the National Basketball Association
  • Philadelphia Phillies of Major League Baseball
  • Philadelphia Flyers of the National Hockey League
• Post-Polio Health International
• Phi, a collegiate secret society at Princeton University

Other uses

• Voiceless bilabial fricative (IPA symbol: ɸ)
• Phi, a village in Sesant, Cambodia
• Phi, ghosts in Thai culture
• Philippines, IOC country code

See also

• Փ, a letter of the Armenian alphabet
• PHI-base (Pathogen–Host Interaction database)
• Kamen Rider 555 or Masked Rider Φ's

• Disambiguation pages