Io

Io most commonly refers to:
 * Io (moon), a moon of Jupiter
 * Io (mythology), daughter of Inachus in Greek mythology, and lover of Zeus who was turned into a cow

Io, IO, iO, I/O, i/o, or i.o. may also refer to:

Fictional elements

 * Scylla Io, one of Poseidon's Marine Generals in the Saint Seiya series
 * A Dungeons & Dragons dragon deity
 * Io Otonashi, a main character in the Japanese manga series Place to Place (Acchi Kocchi)

Gaming

 * iO, a 2014 video game by Gamious
 * IO Interactive, a Danish computer game developer

Music

 * IO (German band), a German band
 * Io (English band), an English band
 * I_o (musician), an American DJ and record producer (1990–2020)
 * i/o (Peter Gabriel album), 2023

Theatre and opera

 * Io (opera), an unfinished acte de ballet (opera) by Jean-Philippe Rameau
 * iO Theater (ImprovOlympic), a theater in Chicago, Illinois, dedicated to improvisational comedy
 * IO West, a Los Angeles theater associated with the Chicago iO

Other uses in arts, entertainment and media

 * EO (original Polish title: IO), a 2022 Jerzy Skolimowski film
 * Io (film), a 2019 Netflix film
 * iO Digital Cable Service, a service offered by Cablevision
 * International Organization, a peer-reviewed journal that covers the entire field of international affairs

Business and economics

 * Industrial and organizational psychology, the field of psychology that studies work, academic, and other organizational issues
 * Industrial organization, the field of economics that studies the strategic behavior of firms, the structure of markets and their interactions
 * Insertion order, a business document specifying the inventory goal of an advertising campaign
 * Integrated operations, a term used in the petroleum industry describing new work processes and ways of working that have been facilitated by modern information and communication technologies
 * Interest-only, a type of collateralized mortgage obligation (CMO) or mortgage-backed security

Language

 * i.o., in illo ordine, Latin phrase meaning "respectively" ("in that order")
 * Io (princely title), a particle of a title used by Moldavian and Wallachian Princes-regnant
 * Ido language (ISO 639-1 language code IO), a constructed language
 * Indirect object, the object that is the recipient of an action (by a verb)
 * Yo, also referred to as Io

Mythology

 * Io (mythology), daughter of Inachus in Greek mythology, and lover of Zeus who was turned into a cow
 * Io, an alternate spelling of the nereid Ino, later known as Leukothea, who in the Odyssey gave Odysseus a veil that allowed him to breathe underwater
 * Io Matua Kore, in some Māori traditions the supreme god

People

 * Io Murota (室田 伊緒), Japanese shogi player
 * Io Sakisaka (咲坂 伊緒), Japanese manga artist
 * Io Shirai (born 1990), Japanese professional wrestler
 * Iou Kuroda (黒田 硫黄), Japanese manga artist
 * iO Tillett Wright (born 1985), American artist, director, photographer, writer, film maker, activist, and actor

Places on Earth

 * British Indian Ocean Territory, ISO 3166-1 alpha-2 code
 * Io (island), an uninhabited islet near Crete, Greece
 * Io, Norway, a village in Alver, Norway
 * Mount Iō (disambiguation), name of several mountains in Japan
 * Indian Ocean

Astronomy

 * Io (moon), a moon of Jupiter
 * 85 Io, an asteroid

Biology and medicine

 * Io (gastropod), a genus of freshwater snail in the family Pleuroceridae
 * Io (plant), a genus of plant in the tribe Senecioneae
 * Automeris io, a moth species in North America
 * Aglais io, the European peacock butterfly
 * Intraosseous infusion, the medical process of introducing medication directly into the bone marrow
 * Hawaiian hawk (io)
 * Immuno-oncology, another term for cancer immunotherapy

Computing

 * .io, the Internet country code top-level domain (ccTLD) for the British Indian Ocean Territory
 * Io (programming language), a pure object-oriented programming language
 * IO.SYS, a system file in Microsoft DOS and Windows 95, 98 and ME
 * Indistinguishability obfuscation, a cryptographic tool to obscure computer code
 * Input/output, the collection of interfaces that different functional units of an information processing system use to communicate with each other

Other uses in science and technology

 * Ionium (symbol Io), a claimed chemical element that was later realized to be thorium-230
 * Iodine monoxide (symbol IO)
 * Image orthicon tube, a TV camera used between 1946 and 1968
 * Infinitely often, a mathematics term sometimes written "i.o."; see an example in set-theoretic limit

Other uses

 * Bureau of International Organization Affairs, in the U.S. Department of State
 * Government Printing Bureau (Macau), known in Portuguese as the Imprensa Oficial or "IO"
 * Indonesian Airlines (IATA airline designator IO)
 * Investigating Officer, a member of law enforcement in the United Kingdom who heads an investigation
 * Mitsubishi Pajero iO, a mini SUV model produced by Mitsubishi Motors
 * IO (Oral-B), a series of electric toothbrushes by Oral-B