It's kind of a nebulous definition, but typically they're games in which you have a variety of options and systems to complete objectives. So things like Deus Ex where you can stealth, fight or hack your way through the plot. Usually the games will have a robust amount of physics or interaction with various objects so there's always a variety of things to do in a level.

Typically, the thing that sets ImmSims apart is that they have a number of interlocking systems that allow the player to solve objectives in different ways.

Stealth, Speech, and Shooting are the usual suspects, with hacking, gunplay and conversation trees well represented in the genre.

But generally, it's a philosophy about designing for extreme player agency.

On one end you have something like, say, Tetris. As the player, you can direct blocks, but you can't stop them from falling. The game gives the player little autonomy to direct. Blocks arrive and the player places them (or doesn't) until the game ends.

On the other, you have something like Dishonored, where you can choose to kill everyone or no one. You can choose to accept and make use of the magical powers available to you - or reject them all and fight with only human strength and your own wits. The world itself then reacts to these choices and the flow of the game changes accordingly.

I think Larian's Baldur's Gate 3 can arguably be called an ImmSim thanks to its insane level of player reactivity.

Basically, if your choices as a player can actually alter the game world and your path through the story, thanks to the emergent interactions of interrelated systems... It's probably an ImmSim.

It's like a FPSRPG