腐烂 (fǔ làn) is a verb meaning 'to rot' or 'to decompose', describing the natural breakdown of organic matter—such as food, plant material, or animal tissue—due to bacterial or fungal activity. The first character 腐 (fǔ) means 'rotten' or 'putrid', often implying decay with odor or discoloration; the second character 烂 (làn) means 'mushy', 'spoiled', or 'thoroughly broken down'. Together, they emphasize advanced, visible decomposition—not just spoilage, but structural disintegration.

This word is commonly used in scientific, agricultural, and everyday contexts when referring to spoiled fruit, decaying leaves, dead animals, or neglected food. It carries a neutral-to-negative connotation and is more formal and precise than colloquial terms like 坏了 (huài le, 'gone bad'). It’s rarely used for abstract concepts—unlike English 'rot' in figurative senses—and almost never applied to inanimate, non-organic objects.