transition element

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of the metallic elements within Groups 3 through 12 in the Periodic Table that have an incomplete inner electron 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry any of the group or metallic elements that have an incomplete inner (d) electron shell; they have multiple valencies and form coloured compounds and stable complex ions; the elements scandium to copper and those lying below them in the periodic table