Elements with partially filled d orbitals are known as transition elements or transition metals.
An element with a partly full d subshell or the ability to create stable cations with an incompletely filled d orbital is defined as a transition element by IUPAC.
Name-Reason:
Transition metals are found in the periodic table between the s-block and p-block elements.
Thus, they are called d-block elements.
Transition metals are unstable metals that display transitional behavior between s and p block elements, thus their name.