A metal is an elemental substance known for its ability to conduct electricity and heat. See the periodic table. Metals have certain characteristic physical properties: they can be shiny, have a high density, are ductile and malleable, have a high melting point, are hard, and conduct electricity and heat well. These properties are mainly because the outer electrons are only loosely held by the atoms, and so form a sort of sea around them.
In chemical terms, a line drawn on the periodic table through the elements B, Si, As, Te, and At roughly separates the metals from the nonmetals. Those elements on the same side of the line as He are nonmetals, while those on the opposite side are metals. Those elements on, or in some cases adjacent to, the line are metalloids or semi-metals. Many of these metalloids, when appropriately doped, form semiconductor materials.
The oxides of metals are basic; those of nonmetals are acidic. The allotropes of metals tend to be lustrous, ductile, malleable, and good conductors, while nonmetals generally speaking are brittle (for solid nonmetals), lack luster, and are insulators.
Although the vast majority of elements are metals, nonmetals are highly abundant.