"Density" (mìdù) is a scientific noun referring to the amount of mass per unit volume of a substance — for example, how tightly packed molecules or particles are in a given space. The character 密 (mì) means 'close-packed', 'dense', or 'tight', while 度 (dù) means 'degree' or 'measure'. Together, they literally mean 'degree of closeness' or 'measure of compactness', which perfectly captures the physical concept of density.

This term appears frequently in physics, chemistry, ecology, and environmental science — especially when discussing population density (e.g., animal or plant density per square kilometer), material properties (e.g., water density), or air density affecting flight or weather. It’s formal and technical; you won’t hear it in casual daily conversation, but it’s essential in academic, research, and textbook contexts. Learners should note that mìdù always functions as a noun and typically follows measure words like 'high' (高) or 'low' (低) when describing values.