Why are transition metals called transition metals?

 They are called that because while the outermost level contains at most two electrons, they are next to outermost main levels have incompletely filled d sub-orbitals, which are filled-up progressively on going across the periodic table from 8 to 18 electrons. The filling of the d sub-orbitals of the transition elements across a row of the periodic table is not always regular.

"The transition metals are called such because an English chemist , Charles Bury described them as the bunch of elements that fill inner shells and bridge the transition between stable states".
-Quora

The correct answer is weight.