A Nipple

(anatomy) The projection of a mammary gland from which, on female mammals, milk is secreted. [from 16th c.]
A mechanical device through which liquids or gases can be passed in a regulated manner. [from 16th c.]
An artificial nipple (definition 1) used for bottle-feeding infants. [from 17th c.]
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.
(now chiefly historical) A perforated segment that fits into the muzzle of a breech-loading gun, on which the percussion cap is fixed. [from 19th c.]
(chiefly US) A short tube threaded at both ends, used as a connector. [from 19th c.]
An internally threaded piece which holds a bicycle spoke in place on the rim. [from 20th c.]