Docosahexaenoic acid: DHA. An essential fatty acid, thought to be important to the development of infants, particularly as regards their eyes and brain. DHA is present in breast milk and has been added to some infant formulas. Postnatal DHA may improve vision and some cognitive functions in infants and toddlers. DHA is an omega-3, polyunsaturated, 22-carbon fatty acid. It is present in abundance in certain fish (such as tuna and bluefish) and marine animal oils.