R (disambiguation)

R is the eighteenth letter of the Latin alphabet.

R or r may also refer to:

Science

Arginine, an amino acid
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

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 (R_∞, R_H), a physical constant relating to energy levels of electrons within atoms
- Rydberg unit of energy (R_y), the energy of the photon whose wavenumber is the Rydberg constant
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)

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

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