R (disambiguation)

R is the eighteenth letter of the Latin alphabet.

R may also refer to:

Science

Spectral resolution ( ${\mathbb {R}}$ ), in astronomy
Gas constant (R), in chemistry
R (cross section ratio), the ratio of hadronic to muonic cross sections
Roentgen (unit) (R), a unit of measurement for ionizing radiation such as X-ray and gamma rays
Rydberg constant, a physical constant relating to energy levels of electrons within atoms
R, one of two chiral center configurations in the R/S system
Side chain (an unspecified group of atoms), in a chemical or structural formula
Orangish or K carbon stars (stellar classification: R)

Arginine, an amino acid
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 population growth rate in the r/K selection theory of ecology
Relatedness, in biology

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. Infrequent button still on some phones today (2017), now mostly used to switch between two calls

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, a film rating in the Motion Picture Association of America film rating system and in the Canadian Home Video Rating System