An obsession is defined as an unwanted intrusive thought, image, or urge that repeatedly enters the person's mind, and that usually causes marked anxiety or distress. Commonly: Contamination from dirt, germs, viruses, bodily fluids, chemicals, sticky substances, dangerous materials; fear of harm; excessive concern with order or symmetry; superstition, fear of 'bad' numbers 'magical' thinking, religious obsessions; 'forbidden' thoughts or images.