# J (disambiguation)

J, or j, is the tenth letter of the English alphabet.
J may also refer to:
* Palatal approximant in the International Phonetic Alphabet
* J, Cyrillic letter Je

## Astronomy

* J, a provisional designation prefix for some objects discovered between May 1 and 15 of a year

## Computing

* J (programming language), successor to APL
* J# programming language for the Microsoft .NET Framework
* J operator, a programming construct
* J (operating system), an operating system for ICL's System 4 series of computers

## Genetics and medicine

* Haplogroup J (mtDNA)
* Haplogroup J (Y-DNA)
* ATC code J Antiinfectives for systemic use, a section of the Anatomical Therapeutic Chemical Classification System

## Mathematics

* J, symbol used to denote the Bessel function
* j, used as the symbol for the imaginary unit (${\sqrt {-1}}$) in fields where i is used for a different purpose (such as electric current)
* j and j$^2$ (or ${\bar {j}}$) are also used for the complex cube roots of unity
* j, a number with the property j $^2$ = +1, used in the definition of the split-complex numbers
* j, the second imaginary unit of a quaternion
* j, an index variable in a matrix
* The j-invariant, a modular function

## Physics and chemistry

* J, Joule, the SI unit of energy
* J, the SI-recommended dimension symbol for luminous intensity, conventionally rendered in a sans-serif typeface
* j →, current density
* second moment of area
* J, name given by Samuel C. C. Ting for the J/ψ particle
* J-coupling, indirect dipole-dipole coupling between two nuclear spins
* J (element), atomic symbol of Jod, German for iodine

## Music

* J, stage name of Jang Ye-eun, a member of the K-Pop group STAYC
* J (Korean singer), a Korean pop singer
* J (musician), a Japanese rock musician
* J., nickname of American White Zombie guitarist Jay Yuenger
* J., stage name of German musician Jaye Muller
* J Mascis, American singer and guitarist of Dinosaur Jr.
* J Records, a record label

### Albums

* J (Jaehyun album), album by South Korean singer Jaehyun

### Songs

* "J", song by Reggie and the Full Effect from the album Last Stop: Crappy Town
* "J", song by Mindless Self Indulgence from the album Frankenstein Girls Will Seem Strangely Sexy

## Film and television

* Agent J, a character in the Men in Black films portrayed by Will Smith
* J, an antagonist in Bobobo
* Kamen Rider J, a Japanese movie
* J, the production code for the 1964 Doctor Who serial Planet of Giants
* Serial Designation "J", one of the antagonists from the webseries Murder Drones (2021-2024)

## Writers

* J (sometimes given as I), pen name of the Danish novelist Cornelia von Levetzow (1836–1921)

## Transportation

* J (Los Angeles Railway)
* J (New York City Subway service)
* J Church (San Francisco Muni)
* J Line (Los Angeles Metro)
* NZR J Class, a 4-8-2 type class of steam locomotive used by New Zealand Railways
* Transilien Line J, a line of the Paris transport network
* , the official West Japan Railway Company service symbol for the Bantan Line
* Saviem J, also known as the Renault J, a range of medium-duty trucks
* "The J", a nickname for the Elgin, Joliet and Eastern Railway, a former railroad

## Other uses

* J, slang for joint (cannabis)
* J, slang for Air Jordan shoes
* J, slang for a jump shot in basketball
* J band (disambiguation)
* J (psychedelic), an entactogenic drug
* Elgin, Joliet and Eastern Railway, a Chicago railway sometimes referred to as "The J"
* J, the international license plate code for Japan
* J, the Canadian postal code for the western and northern regions of Quebec
* J, German mint mark for the city of Hamburg
* J abbreviation for Jahwist, one of the hypothesized major authors of the Torah
* J, an abbreviated title of a High Court judge (in Law Reports, etc.), e.g. Bloggs J
* J. The Jewish News of Northern California, a weekly newspaper serving the Jewish community in Northern California
* J, the narrator of Jerome K. Jerome's novel Three Men in a Boat
* J., Jezper Söderlund, the trance artist
* Lamborghini Aventador J, a 2012 Lamborghini supercar
* J (novel), a 2014 novel by Howard Jacobson
* ʲ, the symbol for Palatalization in the IPA
* Juliet, sometimes used as the unofficial military time zone code for local time
* "J" Is for Judgment, the tenth novel in Sue Grafton's "Alphabet mystery" series, published in 1993
* Guy Big J, range of trucks produced by Guy Motors of England between 1964 and 1978
* [J], code for academic journal in the reference section of an academic work