R (disambiguation)

R is the eighteenth letter of the Latin alphabet. It may also refer to:

Mathematics

• The set of all real numbers, often written as ${\mathbb {R}}$ 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

• 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
• The gas constant in chemistry
• R, a label that denotes one of two chiral center configurations in the R/S system
• In a chemical formula or structural formula, an unspecified group of atoms (often an alkyl group)
• Arginine, an amino acid
• Relatedness, in biology
• r, in ecology, the intrinsic rate of population growth. See r/K selection theory
• 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

Other uses

• R, in American politics, often used as an abbreviation for a Republican (member of the Republican Party)
• R, in law, is an abbreviation meaning The Crown, Rex (King) or Regina (Queen) in Commonwealth nations.
• R. (album) (1998) by R. Kelly
• A film rating in the Motion Picture Association of America film rating system
• Rated R (Queens of the Stone Age album), an album by "Queens of the Stone Age"
• R Cable, a Galician Telecommunications Company
• 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 Rand (currency) of South Africa
• Japan Radio Company's mobile phone of NTT DoCoMo in Japan
• Ryder's NYSE ticker symbol
• R (New York City Subway service)