Second Life is a multiplayer virtual world that allows people to create an avatar for themselves and then interact with other users and user-created content within a multi-user online environment. Developed for personal computers by the San Francisco-based firm Linden Lab, it launched on June 23, 2003, and saw rapid growth for some years; in 2013 it had approximately one million regular users. Growth eventually stabilized, and by the end of 2017, the active user count had fallen to "between 800,000 and 900,000". In many ways, Second Life is similar to massively multiplayer online role-playing video 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).

Philip Rosedale formed Linden Lab in 1999 with the intention of developing computer hardware to allow people to become immersed in a virtual world. In its earliest form, the company struggled to produce a commercial version of the hardware, known as "The Rig", which in prototype form was seen as a clunky steel contraption with computer monitors worn on shoulders. That vision changed into the software application Linden World, in which people participated in task-based games and socializing in a three-dimensional online environment. That effort eventually transformed into the better-known, user-centered Second Life. Although he was familiar with the metaverse of Neal Stephenson's novel Snow Crash, Rosedale has said that his vision of virtual worlds predates that book, and that he conducted early virtual world experiments during his college years at the University of California, San Diego, where he studied physics.

Second Life began to receive significant media attention in 2005 and 2006, including a cover story in BusinessWeek magazine featuring the virtual world and Second Life avatar Anshe Chung. By that time, Anshe Chung had become Second Life's poster child and symbol for the economic opportunities that the virtual world offers to its residents. At the same time, the service saw a period of exponential growth of its user base.

One of the principal developers, Cory Ondrejka, was forced to resign as chief technology officer in December 2007, with Rosedale citing irreconcilable differences in the way the company was run. Nevertheless, the platform continued to grow rapidly, and by January 2008, residents spent a total of 28,274,505 hours "inworld" and on average 38,000 residents were logged in at any moment. The maximum concurrency (number of avatars inworld) recorded was set at 88,200 in the first quarter of 2009.

Second Life was honored at the Technology & Engineering Emmy Awards for advancing the development of online sites with user-generated content in 2008, adding to the media attention. Rosedale accepted the award, although he had announced plans to step down from his position as Linden Lab CEO and to become chairman of Linden Lab's board of directors instead in March 2008. Rosedale announced Mark Kingdon as the new CEO effective May 15, 2008. In 2010, Kingdon was replaced by Rosedale, who took over as interim CEO. After four months, Rosedale abruptly stepped down from the Interim CEO position. It was announced in October 2010 that Bob Komin, Linden Lab's chief financial officer and chief operating officer, would take over the CEO job for the immediate future.

With the platform's failure to continue its high rate of growth after 2009, Linden Lab announced layoffs of 30% of its workforce in 2010. Some 21.3 million accounts were registered by this point, although the company did not make public any statistics regarding actual long-term consistent usage and numbers of dormant accounts.