What element is a transition element?

Answer 1

A transition element is any element that is a transition metal on the Periodic Table.

Added: All elements in group 3 to 12 (period 4, 5 and 6) are transition elements: they have 2 electrons in their most outer shell ( 4th, 5th, and 6th shell) while the shell of 'one level lower' (3rd, ... and so on) is filled up by 1 electron in each of the ten steps higher in atom number)

Atom numbers from 21 to 30 (Sc to Zn) and 39 to 48 (Y to Cd) are the most common transition metals, but also 72 to 80 (Hf to Hg) in period 6, coming right behind the lanthanides (58-71).

[The transition metals reside in the middle of the Periodic Table but do not contain rare earth metals: the Lanthanides and Actinides.]

Answer 2

Any element that isn't part of a representative element group is a transition metal element. This means any element in Groups 0-7 on the Periodic Table.