Noun
(chemistry) Any element in the d block of the periodic table (i.e. groups 3 to 12 inclusive).
(chemistry, dated) The same, but excluding the group 12 elements (i.e. groups 3 to 11 inclusive).
(chemistry, dated) 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.
Source: en.wiktionary.org