Nipple meaning : The projection of a mammary gland from which, on female mammals, milk is secreted. A mechanical device through which liquids or gases can be passed in a regulated manner. An artificial nipple (definition 1) used for bottle-feeding infants. Any small physical protrusion, such as the lumps on the F and J keys on computer keyboards. Any small physical protrusion on an automotive, a machine part or any other part that fits into a groove on another part. A perforated segment that fits into part of the breech of a muzzle-loading gun, on which the percussion cap is fixed. A short tube threaded at both ends, used as a connector. An internally threaded piece which holds a bicycle spoke in place on the rim. To fit (a baby's bottle, etc.) with a nipple. To give one's nipple to (a baby) to allow breastfeeding.