1. A rod or frame of magnetic material that increases the inductance of a coil through which it passes. Cores are used in transformers, electromagnets, and the rotors and stators of electrical machines. It may consist of laminated metal, ferrite, or compressed ferromagnetic particles in a matrix of an insulating binder (dust core).
2. The inner part of a nuclear reactor in which the nuclear reaction takes place.
3. The devices that make up the memory in certain types of computer.
4. The central region of a star or planet.