The psychological condition Derealisation is best described as a subjective episode relating specifically to a person's perception of the world around them. The psychological condition Derealisation is best described as a subjective episode relating specifically to a person's perception of the world around them. The psychological condition Derealisation is best described as a subjective episode relating specifically to a person's perception of the world around them. The psychological condition Derealisation is best described as a subjective episode relating specifically to a person's perception of the world around them.