A transition metal is an element with a partially filled d-subshell in one or more of its oxidation states. Some synonyms for this type of element include, but are not limited to, d-block metal, transition element, and inner transition metal. They are typically characterized by high melting points, varying oxidation states, and the ability to form complex compounds. Most of the elements in the periodic table are transition metals, including iron, copper, nickel, and gold. They are important for many industrial and biological applications, including catalysis, metallurgy, and protein structures.