"녹" (pronounced "nok") refers to the greenish corrosion that forms on metals, particularly copper and its alloys, due to exposure to moisture and air. This process, known as oxidation, results in a layer of 구리산화물 (copper oxide) that can protect the underlying metal from further corrosion. In addition to its occurrence on metals, "녹" can also refer to the green color seen on plants, particularly in the context of 식물 (plants) that thrive in moist environments. This green hue is primarily due to the presence of 엽록소 (chlorophyll), which is essential for photosynthesis, allowing plants to convert sunlight into energy.