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SiN is a first-person shooter video game based on a modified version of the Quake II engine developed by Ritual Entertainment and published by Activision in late 1998. The game was later released over Valve Software's Steam Platform on April 5, 2006, either as a standalone product, or bundled together with its sequel, SiN Episodes.

SiN introduced some features to the FPS genre, such as the ability to knock the weapon out of an opponent's hand and to take area-specific damage from enemies. Although drivable vehicles did not play a big part in the game, there were some sections and levels which required the player to drive certain vehicles, including an ATV, a patrol boat, a forklift and a helicopter. SiN also featured three different types of body armor - for the legs, for the torso and for the head, with each of them depleting separately according to where the player was getting hit.

SiN featured one of the highest levels of interactivity of any first-person shooter at that time. Much of the environment could be interacted with, computer terminals could be manipulated through a DOS-like command prompt, and various objects could be destroyed. Besides, a player's progression through SiN was not entirely linear. Many levels had multiple ways in which to complete them, and actions could trigger drastic changes in future levels. This feature was intended to add a level of replayability to the game and force the player to think before acting. SiN also contained many Easter eggs, more so than most other games, ranging from some fairly obvious signs and graffiti, through to entire secret hidden rooms.

The artificial intelligence of the enemies in the game was on a very high level for its time, with the foes being able to run for cover, call for reinforcements, locate the player throughout the levels, respond to specific scripts etc. However, there were some issues with the game code which prevented the enemies to act completely in the way they were supposed to and unleash their full potential.

Most of the single-player levels were real-life locations like power plants, dams, banks, subway stations, oilrigs etc. Besides them, there were also more sci-fi oriented levels like genetic laboratories, biomech assembly facilities, missile silos, and more. One of the innovative levels featured an entirely underwater landscape in which the player had to find oxygen supply in order to survive. The weapons that the player could obtain in the game ranged from near-future equivalents of present day conventional weapons to experimental devices that required power in order to operate.