One site defines them as: Uncommon and highly valuable metals characterized by their superior resistance to corrosion and oxidation. Included are silver, gold, platinum, palladium, iridium, osmium, rhodium, and ruthenium...
In metallurgy, Precious Metals are usually defined as relatively scarce, valuable metals, in particular gold, silver and members of the platinum group.
In the field of finance, precious metals are specifically gold, silver, platinum and palladium. These four metals are of immense economic importance.