kernel

Concise Oxford English Dictionary © 2008 Oxford University Press:
kernel /ˈkəːn(ə)l/

▶noun

• 1 a softer part of a nut, seed, or fruit stone contained within its hard shell.

  ■ the seed and hard husk of a cereal, especially wheat.

• 2 the central or most important part of something.

  ■ Computing the most basic level or core of an operating system.

– origin OE cyrnel, dimin. of corn1.