a flexible cap on a baby's feeding bottle or pacifier
"The plastic nipple on her new bottle was too hard, so she gently warmed it between her hands to make the feeding more comfortable."
The projection of a mammary gland from which, on female mammals, milk is secreted.
"The baby began to cry when her mother's shirt rode up and exposed her nipple before she could finish feeding."
To fit (a baby's bottle, etc.) with a nipple.
"Please make sure to sterilize and attach a new silicone nipple to the bottle before feeding the infant."