R (disambiguation)

R is the eighteenth letter of the Latin alphabet.

R may also refer to:

Mathematics

• The set of all real numbers, often written as $\Bbb{R}$ or R
• The Pearson product-moment correlation coefficient r in statistics

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

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
• An electronic resistor, e.g. in circuit diagrams
• The R-# refrigerant numbering system
• In fracture mechanics, the ratio between the minimum and maximum stresses

Physics, chemistry and biology

• The spectral resolution in astronomy
• The symbol given to the molar gas constant in chemistry
• A stellar classification for orangish or K carbon stars
• R (cross section ratio), the ratio of hadronic to muonic cross sections
• Röntgen, a unit of measurement for ionizing radiation such as X-ray and gamma rays
• The Rydberg constant, a physical constant relating to energy levels of electrons within atoms
• R, a label that denotes one of two chiral center configurations in the R/S system
• A side chain (an unspecified group of atoms) in a chemical or structural formula
• Arginine, an amino acid
• Relatedness, in biology
• r, representing population growth rate in the r/K selection theory of ecology
• Haplogroup R (mtDNA), a human mitochondrial DNA (mtDNA) haplogroup
• Haplogroup R (Y-DNA), a Y-chromosomal DNA (Y-DNA) haplogroup
• ATC code R Respiratory system, a section of the Anatomical Therapeutic Chemical Classification System
• °R, the symbol for the thermodynamic temperature scale Rankine

Other uses

• R, denoting a member of the Republican Party in American politics
• R, denoting The Crown, Rex (King) or Regina (Queen) in Commonwealth nations
• R. (album) (1998) by R. Kelly
• R (cable operator), a telecommunications company
• A film rating in the Motion Picture Association of America film rating system
• Rated R (Queens of the Stone Age album)
• Rolled R, the alveolar trill of Spanish, Italian, & Arabic languages
• The three Rs of Reading, wRiting and aRithmetic
• In The World Is Not Enough, a character played by John Cleese
• R·Type, an arcade video game
• ®, the registered trademark symbol
• The South African rand currency
• Ryder's NYSE ticker symbol
• R (New York City Subway service)
• R, the military designation for a reconnaissance aircraft.
• R GmbH, a division of the Volkswagen Group, responsible for the so-called R-models.