"Nipple confusion" refers to a situation when a baby becomes confused between different ways of feeding, usually between breastfeeding and bottle-feeding. Full definition
Many breastfed babies don't accept a bottle, however, and there is also the risk of nipple confusion for babies starting to reject the breast if introducing a bottle too early. (easybabylife.com)
Another thing is that they might cause nipple confusion in your baby. (babygearspecialist.com)
By now, most babies are breastfeeding well, and you can usually be less concerned about nipple confusion from taking a bottle of pumped breastmilk. (verywellfamily.com)