The scientific community has two accepted definitions for transition metals: first, it is an element with incomplete d sub-shell atoms, which may or may not result to cations with incomplete d sub-shells. The second defines about it as the elements found in the d block of the periodic table.

Charles Bury first used the word "transition" to describe the elements that undergo a transition series during an electron's change of inner layers. This transition occurs from a stable group of 8 to a group of 18, or from 18 to 32. These elements that underwent transition are now referred to as transition metals or d-block elements.

The atoms of the d-block elements have between 1 and 10 d electrons, with electronic structure written as [ ]ns2(n-1)dm wherein the inner d orbital has more energy than the valence-shell s orbital. While transition elements fall under the d-block, many chemistry textbooks and periodic tables classify La and Ac as d-block elements. This is because they have s2d1 atomic ground-state configurations that are similar to Sc and Y.

The incomplete d sub-shell actually makes d-block elements unique compared to other elements. Partially-filled d shells result to colorful compounds and compounds that are paramagnetic. Many oxidation states also result from the relatively low reactivity of unpaired d electrons. Because of this, d-block elements in aqueous solutions tend to be bright and can range from red, to orange, to yellow (aqueous solutions of Co(NO3)2, K2Cr2O7, and K2CrO4, respectively), and from turquoise, to blue, to purple (aqueous solutions of NiCl2, CuSO4, and KMnO4, respectively).

d-block elements have two or more oxidation states, but the main difference between d-block elements and other elements with regards to oxidation states is that transition metals have oxidation states that are known to have a single atom of the element coupled with one or more electrons that are unpaired.

Transition metal compounds with one or more unpaired d electrons become paramagnetic. Ferromagnetism also occurs when paramagnetic d atoms and their spin vectors are aligned across each other in a material that is crystalline in nature. Some examples of ferromagnetic materials involving transition metals include metallic iron and the alloy called alnico.

d-block elements and compounds have homogeneous and heterogeneous catalytic activity. Catalytic activity refers to their ability to form complexes and adopt more than one oxidation states.

Due to its ability to change oxidation states, transition metal ions are found to be very effective catalysts. At a solid surface, catalysts form bonds between reactant molecules and atoms of the surface of the catalyst. This, in turn, affects the reactants at the catalyst surface by increasing reactant concentration and weakening the reacting molecules' bonds.

d-block elements are metals and can therefore conduct electricity.

Due to metallic bonding by delocalized d electrons, the number of shared electrons increases with cohesion and results, ultimately, to high density, high melting points, and high boiling points for transition metals in general.