难度 (nán dù) refers to the degree or level of difficulty of a task, exam, or problem — it's a measure on a scale. Unlike 困难 (kùn nan, 'difficult/hardship'), which describes the quality of being hard or a general hardship, 难度 is a noun that quantifies how hard something is. It also differs from 难题 (nán tí, 'difficult problem'), which is a specific problem that is hard to solve.