What are the characteristics of the transition elements and why are they called transition elements? Which of the $d$-block elements may not be regarded as the transition elements?

(N/A) Transition elements are defined as elements that have partially filled $d$-orbitals in their ground state or in any of their stable oxidation states.
They are called transition elements because they exhibit properties that are intermediate between the highly reactive $s$-block metals and the less reactive $p$-block elements.
Elements such as $Zn$,$Cd$,and $Hg$ (Group $12$ elements) are not regarded as transition elements because they have completely filled $d$-orbitals ($d^{10}$ configuration) in their ground state as well as in their common stable oxidation states.