Final answer:
The subjective sense that the environment is unreal or altered is called Derealization. It's part of depersonalization/derealization disorder and is distinct from other conditions like fugue, amnesia, and anosognosia.
Step-by-step explanation:
A subjective sense that the environment is changed or unreal is known as Derealization. This sensation is part of a broader category of experiences known as depersonalization/derealization disorder, which involves an individual feeling detached from their surroundings or from reality itself. While depersonalization is more about a sense of detachment from oneself, derealization specifically relates to a sense of unreality in relation to the environment. It is not the same as fugue, amnesia, or anosognosia, which are different mental health concerns.