BOD stands for Biochemical Oxygen Demand, which is a measure of the amount of oxygen required to remove waste organic matter from water in the process of decomposition by aerobic bacteria. BOD provides a measure of the impact of a waste (water) on the oxygen content in a receiving water body. Wastes (organic matter, "food") are broken down by microbial organisms (aerobic bacteria), who require oxygen for this process. BOD is an "estimate" of the "food" available in a sample. The more "food" present in a sample, the more oxygen that will be required and the higher the BOD, the more rapidly oxygen is depleted from a water body. This means less dissolved oxygen (DO) is available to aquatic organisms who then become stressed, suffocate, and die (fish kill) .

BOD is commonly required testing for wastewater treatment plants to measure waste loads, efficiency and effectiveness of BOD removal, control of plant processes, and most importantly the concentration of the effluent that is discharged to water bodies. Knowing BOD value helps municipal authorities determine their biological water quality and industries to assess the impact effluent disposal will have on the immediate environment. Regulations concerning permissible BOD levels for wastewater disposal in industries and municipalities vary from state to state. A higher BOD value indicates a higher level of water pollution, while a lower BOD value indicates less polluted, or cleaner water.