transition element

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of the metallic elements 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. Also called transition metal.

Wiktionary

1. n. 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