A ferromagnet is a piece of ferromagnetic material, in which the microscopic magnetized regions, called domains, have been aligned by an external magnetic field (e.g. from another permanent magnet or an electromagnet) so that the piece itself is a permanent magnet.
The name derives from the Latin ferrum, meaning "iron" (the most well-known ferromagnetic material). Nickel and Cobalt are the other common ferromagnetic elements.
There is much active research in an attempt to find new ferromagnetic materials, especially those which might be light-weight, non-metallic, and capable of remaining ferromagnetic at room temperature.
See ferromagnetism for more information.