A metal is a substance that conducts electricity and heat, is generally malleable and ductile and reflects light (has a metallic lustre).
On contact with acid, a metal gives a salt; on contact with oxygen it gives an oxide (with difficulty for the "noble metals" such as gold, platinum etc.).
There are alkali metals, alkaline earth metals, transition metals (elements with a partially filled d sub-shell), lanthanides or rare earths (with a partially filled 4f sub-shell) and actinides (with a partially filled 5f sub-shell).
Physics The metallic state is defined by its electronic properties due to the metallic bond which is provided by delocalised electrons, unlike the covalent bond (with electrons localised between the ions) or the ionic bond (with electrons localised around the ions). (See band theory).
This state exists for three quarters of the chemical elements, at least for certain allotropic forms: silicon is metallic in the liquid state and covalent in the solid state for example. The electrical conductivity of metals decreases with increasing temperature. At very low temperatures, some metals are superconductors, i.e. they conduct electricity with no energy dissipation. High electrical conductivity is associated with a good thermal conductivity.
Metals have various magnetic properties: paramagnetism, ferromagnetism (iron, nickel), antiferromagnetism (cobalt) etc.
Solid state physics The division into metals, semiconductors and insulators depends on the value of the Fermi energy in relation to the allowed energy bands for the electrons: the top of the valence band, the bottom of the conduction band. The Fermi level of a metal lies in the conduction band.