Thief: The Dark Project is a 1998 first-person stealth video game developed by Looking Glass Studios and published by Eidos Interactive. Set in a medieval steampunk metropolis called the City, players take on the role of Garrett, a master thief trained by a secret society, who while stealing valuable objects, becomes embroiled in a complex plot that ultimately sees him attempting to prevent a great power from unleashing chaos on the world.
Thief was the first PC stealth game to use light and sound as game mechanics, and combined complex artificial intelligence with simulation systems to allow for emergent gameplay. The game is notable for its use of first-person perspective for non-confrontational gameplay, which challenged the first-person shooter market and led the developers to call it a "first-person sneaker", while it also had influences in later stealth games such as Tom Clancy's Splinter Cell and Hitman.
The game received critical acclaim and has been placed on numerous hall-of-fame lists, achieving sales of half a million units by 2000, making it Looking Glass' most commercially successful game. Thief was followed by an expanded edition entitled Thief Gold (1999) which modified certain missions and included a few brand new levels, two sequels - Thief II: The Metal Age (2000), and Thief: Deadly Shadows (2004) - and a reboot of the series - Thief (2014). Thief was one of two games in the series that Looking Glass worked on, before it was forced to close.
Source: Wikipedia, "[en.wikipedia.org/wiki/Thief%3A_The_Dark_Project Thief: The Dark Project]", available under the CC-BY-SA License.