Why elements are called transition?

Answer 1

Transition elements are called transition elements because the electrons in these elements are not in process of filling valence(outermost ) shell instead they are, in case of d-block elements, in process of filling penultimate shell. and in case of f- block elements, electrons are in process of filling next to the penultimate shell.

Answer 2

The transition elements have this name because they represent a transition range in which the lowest total electron energy is achieved by filling the f orbitals of the electron shell below the valence shell rather than filling the rest of the valence shell. After all the noted f orbitals have been filled, the transition is over and addition electrons return to filling the valence shell.

Answer 3

The term "transition metals" is for elements which changes the number of electrons in an inner level of electrons (and thus have a transition) from 8 to 18 or from 18 to 32; I consider this term obsolete and confusing.
Currently the term d-block of the Periodic Table is used.