A metal (from Greek "μέταλλον" – métallon, "mine, quarry, metal") is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually shiny, malleable and ductile. The meaning of the term "metal" differs for various communities (for example, astronomers call for convenience metals everything but hydrogen and helium, see Metallicity). Many elements and compounds that are not normally classified as metals become metallic under high pressures (see Nonmetal: Metallic allotropes.)