Transition elements, also known as transition metals, are chemical elements that have partially filled d orbitals. According to IUPAC, transition elements are elements that have a d subshell that is partially filled with electrons, or an element that has the ability to form stable cations with an incompletely filled d orbital. In general, any element that corresponds to the d-block of the modern periodic table (which consists of groups 3-12) is considered to be a transition element. The transition elements occupy the middle portions of the long periods of the periodic table of elements between the groups on the left-hand side and the groups on the right. They form Groups 3 (IIIb) through 12 (IIb) and are included in the set of 30 elements often called the d-block transition metals.

The electronic configuration of transition elements is different from that of non-transition elements. Transition elements have valence electrons in two shells instead of only one, which gives them unique chemical properties. Without exception, the elements of the main transition series form stable compounds in two or more formal oxidation states. The transition metals may be subdivided according to the electronic structures of their atoms into three main transition series, called the first, second, and third transition series, and two inner transition series, called the lanthanoids and the actinoids.

It is important to note that the elements mercury, cadmium, and zinc are not considered transition elements because of their electronic configurations, which correspond to (n-1)d10 ns2. The rest of the d-block elements are somewhat similar in properties, and this similarity can be observed along each specific row of the periodic table. Transition elements have many important uses, such as in the production of steel, magnets, and electronics.