Nub (noun)

1. A small projection or protuberance.
2. A small, essential, or noteworthy part or aspect of something.
3. A short, stubby piece of wood or other material used to support or hold something.

Origin:

Of uncertain origin, possibly from the latin word 'nubere' meaning to cover or cloud over.

Examples:

1. The sculpture was missing its nose, leaving only a small nub.
2. The nub of the argument was that the policy was discriminatory.
3. The nub of the matter is that we need to act quickly.
4. The nub of the pen was chewed, revealing the ink cartridge.
5. He used a nub of charcoal to sketch the outline of the picture.