nicotinamide adenine dinucleotide: a coenzyme present in most living cells and derived from the B vitamin nicotinic acid; serves as a reductant in various metabolic processes
coenzyme: a small molecule (not a protein but sometimes a vitamin) essential for the activity of some enzymes
cell: (biology) the basic structural and functional unit of all organisms; they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals