R is the eighteenth letter of the Latin alphabet. R or r may also refer to: Science 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 (), 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 or R, the set of all real numbers Pearson product-moment correlation coefficient (r), in statistics Ricci curvature 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 Arts and entertainment Music Ritardando, a term for slowing the tempo down gradually R. (R. Kelly album), a 1998 album