Second Life is an online multimedia platform that allows people to create an avatar for themselves and then interact with other users and user-created content within a multiplayer online virtual world. Developed and owned by the San Francisco–based firm Linden Lab and launched on June 23, 2003, it saw rapid growth for some years and in 2013 it had approximately one million regular users. Growth eventually stabilized, and by the end of 2017 the active user count had declined to "between 800,000 and 900,000". In many ways, Second Life is similar to massively multiplayer online role-playing games; nevertheless, Linden Lab is emphatic that their creation is not a game: "There is no manufactured conflict, no set objective".

The virtual world can be accessed freely via Linden Lab's own client software or via alternative third-party viewers. Second Life users, also called 'residents', create virtual representations of themselves, called avatars, and are able to interact with places, objects and other avatars. They can explore the world (known as the grid), meet other residents, socialize, participate in both individual and group activities, build, create, shop, and trade virtual property and services with one another.

The platform principally features 3D-based user-generated content. Second Life also has its own virtual currency, the Linden Dollar (L$), which is exchangeable with real world currency.

Second Life is intended for people ages 16 and over, with the exception of 13–15-year-old users, who are restricted to the Second Life region of a sponsoring institution (e.g., a school).