transition element

Short definition from Wiktionary

Noun

1. 1
   Any element in the d block of the periodic table (i.e. groups 3 to 12 inclusive).
2. 2
   The same, but excluding the group 12 elements (i.e. groups 3 to 11 inclusive).
3. 3
   Any of the group of metallic elements that have an incomplete inner (d) electron shell, having multiple valencies and forming coloured compounds and stable complex ions.