Biological Oxygen Demand (BOD) is a critical indicator used to assess the level of organic pollution in aquatic ecosystems. It measures the amount of oxygen consumed by microorganisms while decomposing organic matter over a specified period, typically five days. A high BOD value indicates higher levels of organic material and potential pollution, leading to oxygen depletion, which can adversely affect aquatic life, thus serving as an essential criterion in water quality monitoring and environmental management.