What does IO stand for? In computers and electronics, IO (often written I/O) stands for Input/Output, the flow of data into and out of a device or program. A keyboard sends input, a screen shows output, and a hard drive does both.
Outside of hardware, this pair of letters leads several other lives. The ending .io on a website or a browser game is a country code domain, not an abbreviation of a phrase. In advertising, an IO is a signed order for ad space, and in hospitals it can describe a way of giving fluids through the bone. A game lobby, an ad contract and an ER chart each point to a different reading.
Quick facts
Stands for
Input/Output (computing)Other meanings
.io web domain, insertion order, intraosseous, industrial-organizationalType
InitialismHow to say it
"eye oh," one letter at a timeAlso written
I/O, IO, i/oUsed in
Hardware specs, coding, game names, ad contracts
What does IO stand for in computing?
In tech, the letters break down simply:
| Letter | Word | What it covers |
|---|---|---|
| I | Input | Anything a device receives: key presses, mouse clicks, sensor readings, files being read |
| O | Output | Anything a device sends out: pixels on a screen, sound, printed pages, files being written |
Engineers talk about I/O ports (the physical sockets on a laptop or console), I/O devices (keyboards, printers, speakers) and I/O speed, which is how fast a drive or network can move data. When a program feels slow because it is waiting on a disk or a network, programmers call it "I/O bound." For the ports themselves, see USB and HDMI, two of the most common.
What does IO stand for in games?
Many free browser games have names that end in .io, and players call the whole category ".io games." Here the letters do not stand for a phrase. The ending .io is the internet country code for the British Indian Ocean Territory, the same way .us belongs to the United States.
Tech startups liked .io because it looks like I/O, and short names were easy to get. Game makers followed, and a wave of simple multiplayer games, where you steer a dot, a snake or a tank around a shared map, picked the ending. The style stuck so well that "io game" now describes the type of game: quick to load, played in a browser, many players at once.
A friend texts: "Found a new io game, play during lunch?" They mean a light, browser based multiplayer game, not anything about input or output.
IO meanings outside tech
| Field | Stands for | Example |
|---|---|---|
| Advertising | Insertion order | "Send the signed IO before the campaign starts." |
| Medicine | Intraosseous | "The team placed an IO line when no vein could be found." |
| Psychology | Industrial-organizational (I-O) | "She studies I-O psychology to work on hiring." |
| Space | Io, a moon of Jupiter | "Io has active volcanoes." (A name, not an abbreviation) |
| Military and government | Information operations | "The report covered IO in modern conflicts." |
An insertion order is the contract between an advertiser and a publisher that lists the ad, the dates and the placement. In emergency care, an intraosseous line delivers fluids or medicine into the bone marrow when veins are hard to reach.
How to say and write IO
Say the letters separately: "eye oh." Nobody blends it into one syllable, so IO lands in the initialism camp. (The moon Io is the exception; it is a proper name, said "EYE-oh" as well, but it is not short for anything.) If you want the background on that difference, read what is an acronym.
- I/O with a slash is the classic tech spelling, common in manuals and spec sheets.
- IO without the slash shows up in code, file names and job titles.
- .io in lowercase with a dot is the web domain.
- I-O with a hyphen is the usual form for the psychology field.
The spoken "eye" sound calls for "an" in front: "an I/O error," "an IO line."
IO in real sentences
- "The board has eight digital I/O pins for buttons and lights."
- "Disk IO spiked when the backup started."
- "My cousin spends every evening on some .io game with a growing snake."
- "Marketing is still waiting for the client to sign the IO."
- "The paramedic was trained to place an IO needle in the shin."
An error message: "I/O device error. The request could not be performed." This means the computer could not read from or write to a drive, card or cable. Checking the connection is a good first step.
Look-alikes: IO, iOS, IP and OI
A few short forms sit close to IO and trip people up:
- iOS is a phone operating system name. The letters "io" inside it are not I/O.
- IP usually means Internet Protocol, the address system for devices on a network.
- API is a set of rules programs use to talk to each other, often to request input and return output.
- OI reversed is not a standard tech term, though it appears as a casual greeting in some places.
You will find more of these letters on our tech acronyms page.
Questions people ask
What does IO mean in a game title?
It refers to the .io web address the game is hosted on. That ending is a country code domain, and it became the label for simple multiplayer browser games.
Is I/O the same as IO?
Yes, in computing they mean the same thing, Input/Output. The slash version is more traditional, and the plain version is common in code.
What is an I/O error?
It means a device could not complete a read or write. Loose cables, a failing drive or a damaged memory card are common causes.
What does IO stand for in business?
In advertising and media buying, IO stands for insertion order. It is the signed agreement that sets what ads run, where and when.
Is IO an acronym?
Not really. Speakers pronounce both letters one by one, which puts IO in the initialism group, and the domain is read "dot eye oh."