An intermetallic (also called intermetallic compound, intermetallic alloy, ordered intermetallic alloy, long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic.

The term "intermetallic compounds" applied to solid phases has long been in use. However, Hume-Rothery argued that it misleads, suggesting a fixed stoichiometry and a clear decomposition into species.

In 1967 Gustav Ernst Robert Schulze [de] defined intermetallic compounds as solid phases containing two or more metallic elements, with optionally one or more non-metallic elements, whose crystal structure differs from that of the other constituents. This definition includes:

The definition of metal includes:^{[citation needed]}

Homogeneous and heterogeneous solid solutions of metals, and interstitial compounds such as carbides and nitrides are excluded under this definition. However, interstitial intermetallic compounds are included, as are alloys of intermetallic compounds with a metal.^{[citation needed]}

In common use, the research definition, including post-transition metals and metalloids, is extended to include compounds such as cementite, Fe₃C. These compounds, sometimes termed interstitial compounds, can be stoichiometric, and share properties with the above intermetallic compounds.^{[citation needed]}

The term intermetallic is used to describe compounds involving two or more metals such as the cyclopentadienyl complex Cp₆Ni₂Zn₄.

A B2 (also known as cesium chloride structure type) intermetallic compound has equal numbers of atoms of two metals, such as aluminium-iron, and aluminium-nickel, arranged as two interpenetrating simple cubic lattices of the component metals.