R (disambiguation)

R, or r, is the eighteenth letter of the English alphabet.

R or r may also refer to:

Biology and medicine

 * Arginine, an amino acid abbreviated as Arg or R
 * ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
 * Coefficient of relationship (r), in biology
 * Effective reproduction number (R), the number of cases generated by one case in the current state of a population in epidemiology
 * Basic reproduction number (R0), the expected number of cases directly generated by one case
 * Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
 * Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
 * Net reproduction rate (R0), the average number of offspring that would be born to a female given conforming conditions
 * r, the population growth rate in the r/K selection theory of ecology

Astronomy

 * Spectral resolution ($$\mathbb{R}$$), in astronomy
 * Orangish or K carbon stars (stellar classification: R)

Physics

 * R (cross section ratio), the ratio of hadronic to muonic cross sections
 * Electrical resistance (R)
 * Roentgen (unit) (R), a unit of measurement for ionizing radiation such as X-ray and gamma rays
 * Rydberg constant (R∞, RH), a physical constant relating to energy levels of electrons within atoms
 * Rydberg unit of energy (Ry), the energy of the photon whose wavenumber is the Rydberg constant

Temperature scales

 * Rankine scale (°R, °Ra)
 * Réaumur scale (°Ré, °Re, °r)
 * Rømer scale (°R, °Rø)

Chemistry

 * Gas constant (R), in chemistry
 * R, one of two chiral center configurations in the R/S system
 * Alkyl group, commonly marked as R in structural formulas
 * Side chain (an unspecified group of atoms), in a chemical or structural formula

Mathematics

 * r is the symbol for the radius of a circle
 * $$\mathbb{R}$$ or R, the set of all real numbers
 * Pearson product-moment correlation coefficient (r), in statistics
 * Ricci curvature
 * $Γ$, the gamma function

Technology and engineering

 * R-value (insulation), a non-SI measure of thermal resistance used in housing insulation
 * R-value (soils), property of soils used in pavement design
 * Resistor, an electronic component
 * R-, a refrigerant numbering system
 * R, the ratio between the minimum and maximum stresses in fracture mechanics
 * R- or recall-button, now mostly used to switch between two calls

Computing

 * R (programming language), an environment for statistical computing and graphics
 * R (complexity), the set of all recursive languages
 * IBM System R, an IBM database system
 * RequestBot, a service in QuakeNet's IRC services

Music

 * Ritardando, a term for slowing the tempo down gradually
 * R. (R. Kelly album), a 1998 album by R. Kelly
 * R° (Ruratia album), a 2002 album by Rurutia
 * R (single album), a 2021 single album by Rosé
 * Rated R (Queens of the Stone Age album), 2000 album also called R
 * R (jp), 2005 J-Pop album by Ryoko Shiraishi

Film

 * R (film), a 2010 film by Danish screenwriter and film director Tobias Lindholm
 * R, a character played by John Cleese in The World Is Not Enough
 * R, the protagonist of Warm Bodies, based on the book of the same name
 * R, a film rating in the Motion Picture Association of America film rating system and in the Canadian Home Video Rating System

Television

 * Rajawali Televisi (RTV), television in Indonesia
 * R, the production code for the 1965 Doctor Who serial The Chase

Video games

 * R·Type, an arcade video game
 * R: Rock'n Riders, in the list of PlayStation games (M–Z)

Organisations

 * The Crown, Rex (King) or Regina (Queen) in Commonwealth nations
 * Ryder (NYSE ticker symbol)
 * R (cable operator), a telecommunications company
 * R, denoting a member of the US Republican Party

Transportation

 * Nissan Skyline GT-R, commonly referred to as the "R" among Japanese speakers
 * R (New York City Subway service)
 * Rinkai Line, a railway line operated by the Tokyo Waterfront Area Rapid Transit, labeled
 * Transilien Line R, a line of the Paris transport network
 * Reconnaissance aircraft (military designation: R)
 * Restricted airspace
 * Volkswagen R, the performance division of Volkswagen
 * The official West Japan Railway Company service symbol for:
 * JRW kinki-R.svg Hanwa Line.
 * JRW hiro-R.svg San'yō Main Line.

Other uses

 * Registered trademark symbol (®)
 * South African rand (R), a currency
 * Right (direction)
 * Rolled R (IPA: r), the alveolar trill of Spanish, Italian, & Arabic languages
 * UTC−05:00 (military time zone code: Romeo)
 * Я, a Cyrillic letter similar to R