Immersive theatre has many definitions, but all of them involve some sort of active involvement from the audience that goes beyond just sitting and watching in silence. A lot of the work I did as a theatre director used immersive techniques.

Some examples:
Verrassing was a project I did that added an immersive touch to a location based theatre festival. Several actors who weren’t immediately recognizable as such would interact with the audience as they went from location to location, trying to trigger them to think about their role in the group of spectators and challenging their role as a passive audience member.
Als een Vis op het Water was a theatre production developed to immerse the audience into a newly built community center. Spectators were led through the building by an actor who asked them to actively engage in the different rooms and stories, and to develop their own personal story.
Save the World is playable as an individual inversed scavenger hunt, but some editions have made use of actors to guide the players on their way. Strange encounters that would nudge players to make specific choices, or make them reconsider the choices they’d already made.