Noun

1. The projection of a mammary gland from which, on female mammals, milk is secreted.
2. An artificial nipple (definition 1) used for bottle-feeding infants.
3. A mechanical device through which liquids can be passed in a regulated manner.
4. A short tube threaded at both ends, used as a connector.

   Turn the nipples on to the bicycle spokes only one turn, for each spoke on the new wheel all the way around until they are all snug, then check alignment.

5. Any small physical protrusion, such as the lumps on the F and J keys on American keyboards.
6. Any small physical protrusion on an automotive, a machine part or any other part that fits into a groove on another part.