The players ability to rotate the virtual world around them in relation their real world. Tends to be disorienting, but sometimes essential. Often implemented to work with non full-360 6dof tracking solutions, like the PSVR, and is helpful for accessibility.
While it can be helpful for such accessibility situations, design wise, turning does a lot to remove presence, and can be severely disorienting. Like the 30 degree rule in film-editing, moving to a new location - but not that new of a location can be more disorienting than teleporting somewhere new entirely.