The .io genre spawned directly from a single game, Agar.io, made by Brazilian developer Matheus Valadares.
While that original game used .io in its original sense, as a top level domain name, subsequent .io games have used the term to signify a certain structural kinship. As a result, an .io game can loosely be described as a simple massively multiplayer online action game with a pronounced 'survival of the fittest' angle.
These games are typically quite stripped back, and are often playable through a website and on smart devices. Yep, they've become pretty business on mobile.
The .io genre has grown to a considerable size, with dozens if not hundreds of games taking up Agar.io's mantle. Along the way this fledgling genre has spread its wings and adopted a range of other influences and gameplay mechanics.
With that in mind we thought now would be a good time to round up some of the best and most popular .io games on the iOS App Store. Is you favourite missing from our list? Let us know in the comments below.