A feeling caused by anxiety where places that should be familiar seem 'off' in some way. Derealisation (or derealization) extremely hard to describe to someone who has never experienced it before, although if you're here I imagine you're experiencing it right now. It's often described as like a veil has been put over you that dull all your senses and your surroundings, although personally I find it's like everything is too 'vivid'. It normally goes away completely by itself when whoever experiences when they stop thinking about it. See deperonalisation