Natural killer cells: Specialized cells of the immune system that kill bacteria, viruses, and other foreign substances in the body.

Neoplasm: Any new abnormal growth. A neoplasm may be malignant or benign.

Neuropsychiatric: Having to do with physical and mental diseases of the nervous system (the brain, spinal cord, and nerves).

Neutropenia: A reduction in white blood cell count.

Nevi: Plural of nevus.

Nevus: The medical term for a noncancerous growth on the skin, such as a mole.

Nodule: Solid, raised bumps on the skin, greater than 10 millimeters in diameter.

Nurse: A professional who is trained to assist the doctor or to provide direct care for the patient under the doctor's supervision. A nurse's responsibilities vary depending on his or her level of training.