What is the term for a diminished amount of urine formation?

The term for a diminished amount of urine formation is indeed "oliguria." This condition is characterized by a significant decrease in urine output, often defined as producing less than 400 milliliters of urine per day in adults. Oliguria can be a sign of various medical conditions, such as dehydration, kidney disease, or urinary obstruction, indicating the kidneys are not filtering adequately.

In contrast, anuria refers to an even more severe reduction in urine output, where the kidneys fail to produce more than 100 milliliters per day. Polyuria is characterized by increased urine output, often more than 2.5 liters per day, which can occur in conditions like diabetes mellitus. Hematuria indicates the presence of blood in the urine, which is a different condition related to urinary tract issues or kidney problems. Thus, oliguria specifically describes a notable decrease in urine production, making it the correct answer to the question.