Kernel

noun

1. The core, center, or essence of an object or system.

   "the kernel of an argument"

2. The central (usually edible) part of a nut, especially once the hard shell has been removed.
3. A single seed or grain, especially of corn or wheat.
4. The stone of certain fruits, such as peaches or plums.
5. A small mass around which other matter is concreted; a nucleus; a concretion or hard lump in the flesh.