Answer:
Any of the metallic elements that have an incomplete inner electron shell (d sub-shell) and that serve as transitional links between the most and the least electropositive in a series of elements. They are characterized by multiple valences, colored compounds, and the ability to form stable complex ions. Also called transition metal.
Technically Scandium and Zinc are not transition metals as they form only one ion which leads to Scandium having a completely empty d-sub shell and in Zinc; a completely full d sub-shell.
The technical definition of a transition metal is when the metal contain at least one unpaired d-sub shell in at least one ion.