In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:

Jensen has reviewed the history of the terms transition element (or metal) and d-block. The word transition was first used to describe the elements now known as the d-block by the English chemist Charles Bury in 1921, who referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32.

In the d-block the atoms of the elements have between 1 and 10 d electrons.

With a few minor exceptions, the electronic structure of transition metal atoms can be written as [ ]ns²(n-1)d^m, where the inner d orbital has more energy than the valence-shell s orbital. In divalent and trivalent ions of the transition metals, the situation is reversed such that the s electrons have higher energy. Consequently, an ion such as Fe²⁺ has no s electrons: it has the electronic configuration [Ar]3d⁶ as compared with the configuration of the atom, [Ar]4s²3d⁶.