Which word describes a condition of diminished urine formation?

The term that describes a condition of diminished urine formation is "oliguria." This medical term specifically refers to the reduced output of urine, typically defined as producing less than 400 milliliters of urine per day in adults. Oliguria can be an important clinical sign, indicating potential issues with kidney function, dehydration, or other underlying health problems that require medical attention.

The other terms listed do not relate to this specific condition. "Nocuous" refers to something that is harmful or injurious; "nidus" pertains to a focal point of infection or a nest-like structure in medical contexts; and "nosocomial" describes infections that are acquired within a hospital setting. Therefore, "oliguria" is the only choice that accurately conveys the concept of decreased urine production.