What is the meaning of Transition_element?

Any element in the d block of the periodic table (i.e. groups 3 to 12 inclusive).

The same, but excluding the group 12 elements (i.e. groups 3 to 11 inclusive).

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: wiktionary.org