You no doubt heard of it, you might even recognize it, but can you explain what it actually is? It is rather simple, really. What makes these games distinct is that they try to account for everything the player might want to do. When there is a locked door you can pick it, break it, steal the key, threaten the guard to open it or just ignore it and go around. In practice there are always limitations, because development time and budget is not infinite.
However, the name itself is just a product of marketing. Truly important is only the "Sim" part, because these games are simulations. But unlike the classic simulators, which just focus on physics of planes and such, these simulate whole worlds. They are the ultimate simulation and the "Immersive" part is redundant, because immersion comes naturally with good simulation. Can you guess what other buzzwords we get with good simulation? It is “Emergent Gameplay” and “Emergent Storytelling/Narrative”.
So, what does it mean that gameplay is emergent? It is very close to saying unintended. It usually takes form of a developer introducing a challenge in their game and the player finding other solution than the one developer intended. It makes you feel clever, it makes repeated playthroughs more fun and again it is a product of a good simulation. When you run around with a bazooka and get stopped by a wooden door, that breaks immersion. I blew up that barrel over there, why can’t I blow up this door? Bad simulation.
Emergent narrative it is pretty much the same, but I think it is overused. Looks like almost every open ended game gets labeled with emergent narrative, but most of them don’t deserve it. For example, let’s look at Minecraft. It is true that nothing you do in that game was precisely planned out by the developer, but is “I dug underground, found iron in a cave then went back home to craft a better pickaxe.” really good enough to be called a “story”? A story doesn’t have just the player doing stuff. It has other characters, with their own motivations that come in conflict.
Games that have emergent narrative worth talking about have NPCs that can affect the world just like the player can. A good example are games like the Civilization series. There the player meets other nations with their own leaders, that have their own goals. It may happen that the player is close to a different nation, so they compete for land. The player makes some friends and gets told their neighbor is up to something. Maybe then the player denounces the wrong nation and suddenly their neighbor has armies on the border. War starts but the advance is slow enough that the player manages to grab an important resource and upgrade their units with gold from trading with their friend, turning the war around and capturing the invaders capital. A peaceful period follows and both the player and their friend grow bigger. But there can be only one victor, so the player deems it necessary to stop their friend from winning and so attacks. Now that is a story.