a small group of indispensable persons or things

the center of an object

the central part of the Earth

the choicest or most essential or most vital part of some idea or experience

a cylindrical sample of soil or rock obtained with a hollow drill

the central meaning or theme of a speech or literary work

(computer science) a tiny ferrite toroid formerly used in a random access memory to store one bit of data; now superseded by semiconductor memories

the chamber of a nuclear reactor containing the fissile material where the reaction takes place

a bar of magnetic material (as soft iron) that passes through a coil and serves to increase the inductance of the coil