An immersive sim (simulation) is a video game genre that emphasizes player choice. Its core defining trait is the use of simulated systems that respond to a variety of player actions, which combined with a comparatively broad array of player abilities allow the game to support varied and creative solutions to problems as well as emergent gameplay beyond what has been explicitly designed by the developer. This definition is not to be confused with game systems which allow player choice in a confined sense or systems which allow players to easily escape consequences of their choices.

Immersive sims by definition allow for multiple approaches and typically incorporate elements of multiple genres including role-playing games, stealth, first-person shooters, platform games, survival horror and adventure games. Although they typically have smaller worlds than open world games, they also generally allow for open-ended gameplay, allowing the player to progress in any order and pursue side missions alongside any main story missions. Immersive sims are generally compared to games developed by Looking Glass Studios. The first such game generally considered an immersive sim is Ultima Underworld: The Stygian Abyss, and other early examples include Thief: The Dark Project and Thief II: The Metal Age, System Shock and System Shock 2, as well as the first Deus Ex.

The term "immersive sim" emerged in the 2000s and began to be used regularly in the second half of the 2010s, often applied retroactively to the earlier entries in the genre. It may also be used to describe the set of game design principles behind the immersive sim genre, which uses interacting, reactive and consistent game systems to create emergent gameplay and a sense of player agency.

Immersive sims typically task the player to make their way through levels and complete missions, but do not enforce the means by which the player does this. A common example would be where the player-character must get past a guard. The choice of how to do this would be up to the player: they may attempt to sneak around; use parkour or other similar abilities, aided with some equipment, to slip around them; find small passageways that allow them to get around the guard; create a distraction that draws the guard away from their post; convince or bribe the guard to ignore them; or simply attack and kill or disable the guard directly. The choices may be limited by the player-character's abilities and current inventory, and there may be consequences of the player's choice. For example, killing or disabling the guard could leave the guard's body to be found later, raising the alert level of other guards. However, this element of consequence can be seen as a negative to players if taken throughout the game. For example, Dishonored introduced a "chaos" system that adjusted how enemy guards would behave based on how much violence and disturbance a player had used earlier in the game so as to encourage players to try different tactics to avoid making future encounters harder. This system was removed for Dishonored: Death of the Outsider, as well as the addition of optional quests to encourage alternate approaches to missions.

Warren Spector, part of Looking Glass Studios, said that immersive sims create the feeling that "you are there, nothing stands between you and belief that you're in an alternate world". Many of the key developers of immersive sims compare them to tabletop Dungeons & Dragons games hosted by a good gamemaster, or to live action role-playing games, in that there are a set of rule systems to keep it a game, but the game will react to the players' actions rather than force the player to conform to a specific action. Spector is credited with the term "immersive sim" in a post-mortem he wrote on the development of Deus Ex in 2000, although Spector himself attributes it to his Looking Glass colleague Doug Church instead.

Mark Brown of the YouTube series Game Maker's Toolkit identified that a key differentiating feature of immersive sims is that they do not readily use scripted or fixed events. Instead, they use a consistent series of rules and systems throughout the game. These consistent systems then can be exploited by the player to complete objectives in unique and unpredictable ways, with the game reacting to the player's decisions. Brown uses the example of being able to fire rope arrows (to climb on) at any wooden surface in the original Thief: The Dark Project (1998), while the 2014 Thief game limited what locations these could be used, removing the immersive sim elements. Rick Lane of PC Gamer noted that while earlier games in The Elder Scrolls series were not immersive sims, The Elder Scrolls IV: Oblivion (2006) transitioned the series to an immersive sim.

Warren Spector considered Ultima VI: The False Prophet (1990) the first game to have an immersive sim mentality as while played from a top-down view, it relied less on events and planned-out puzzles, and instead provided the rulesets and systems through its living world to allow players to craft their own solutions to situations. Spector described one playtesting example from Ultima VI that he considered the genesis of the immersive sim genre, in which a playtester lacked a magical spell needed by his party to pass by a closed gate, and instead used a pet mouse character to sneak through small spaces and access the necessary controls to open the gate, something none of the developers had anticipated.

Ultima Underworld: The Stygian Abyss (1992) is considered the first game to demonstrate the necessary elements of an immersive sim according to Spector and others. It built upon Ultima VI's gameplay and added in the first-person perspective, predating Wolfenstein 3D, the game that first popularized first-person shooters, by a few months. The first-person view helped to cement the impression that the player was part of the game's world that they had full control of, and completing the impression of immersion. Spector recalled that he had thought to himself "Do you not realize that the entire world just changed?" on seeing the initial demo for Ultima Underworld. Other early examples include System Shock (1994) and its sequel System Shock 2 (1999), Thief: The Dark Project (1998) and its sequel Thief II: The Metal Age (2000), Deus Ex (2000), and Arx Fatalis (2002). However, at the same time, more action-oriented games with strong narrative elements that followed from Wolfenstein 3D, like Doom (1993) and Half-Life (1998), drew larger commercial sales, making it difficult to gain publisher interest.