Ferromagnetism
Definition
The strong magnetism of iron, nickel, and cobalt — their atomic magnets spontaneously align in regions called domains.
Technical definition
The exchange interaction aligns electron spins below the Curie temperature; domains grow or rotate in an applied field, giving large permeability and hysteresis.
Example
A fridge magnet is ferromagnetic — its domains were aligned once at the factory and stay that way, holding up notes for years.
Common confusion
Not all metals are ferromagnetic. Aluminum and copper are essentially non-magnetic — a magnet won't stick to them, and that is completely normal.