树木 (shù mù) is a formal, collective noun meaning 'trees' — referring to trees as a category or group, not individual trees. The character 树 (shù) means 'tree' on its own, while 木 (mù) means 'wood' or 'tree' in a more general, botanical sense; together, they form a compound that emphasizes the natural, living plant category. This term is commonly used in written Chinese, scientific contexts, environmental discussions, and official documents — for example, when talking about forest conservation, urban greening, or ecological surveys.

Unlike the colloquial word 树 (shù), which can refer to one or more trees depending on context, 树木 always implies plurality and abstraction — you wouldn’t say *一棵树木* (one tree) because it’s inherently collective and uncountable. It often appears with measure words like 类 (lèi, 'type') or 棵 (kē) only when specifying countable instances, but then the phrasing shifts (e.g., 几种树木 'several types of trees').