An online game is a video game that is either partially or primarily played through the Internet or any other computer network available.[1] Online games are ubiquitous on modern gaming platforms, including PCs, consoles and mobile devices, and span many genres, including first-person shooters, strategy games, and massively multiplayer online role-playing games (MMORPG).[2] In 2019, revenue in the online games segment reached $16.9 billion, with $4.2 billion generated by China and $3.5 billion in the United States.[3] Since the 2010s, a common trend among online games has been to operate them as games as a service, using monetization schemes such as loot boxes and battle passes as purchasable items atop freely-offered games.[4][5] Unlike purchased retail games, online games have the problem of not being permanently playable, as they require special servers in order to function.

The history of online games dates back to the early days of packet-based computer networking in the 1970s,[7] An early example of online games is MUDs, including the first, MUD1, which was created in 1978 and originally confined to an internal network before becoming connected to ARPANet in 1980.[21] Commercial games followed in the next decade, with Islands of Kesmai, the first commercial online role-playing game, debuting in 1984,[21] as well as more graphical games, such as the MSX LINKS action games in 1986,[22] the flight simulator Air Warrior in 1987, and the Family Computer Network System's online Go game in 1987.[23]

Xbox Live was launched in November 2002. Initially the console only used a feature called system link, where players could connect two consoles using an Ethernet cable, or multiple consoles through a router. With the original Xbox Microsoft launched Xbox Live, allowing shared play over the internet. A similar feature exists on the PlayStation 3 in the form of the PlayStation Network, and the Wii also supports a limited amount of online gaming. Nintendo also had a network, dubbed "Nintendo Network", that fully supported online gaming with the Wii U and Nintendo 3DS. With the launch of the Nintendo Switch, Nintendo launched the Nintendo Switch Online service to replace the older Nintendo Network.

As the World Wide Web developed and browsers became more sophisticated, people started creating browser games that used a web browser as a client. Simple single player games were made that could be played using a web browser (most commonly made with web technologies like HTML, JavaScript, ASP, PHP and MySQL).

The development of web-based graphics technologies such as Flash and Java allowed browser games to become more complex. These games, also known by their related technology as "Flash games" or "Java games", became increasingly popular. Games ranged from simple concepts to large-scale games, some of which were later released on consoles. Many Java or Flash games were shared on various different websites, bringing them to wide audiences.[36] Browser-based pet games are popular among the younger generation of online gamers. These games range from gigantic games with millions of users, such as Neopets, to smaller and more community-based pet games.

More recent browser-based games use web technologies like Ajax to make more complicated multiplayer interactions possible and WebGL to generate hardware-accelerated 3D graphics without the need for plugins.