an intermetallic compound
Sometimes two metals do not just dissolve in each other. They bond in a fixed recipe to make a brand-new crystalline compound with its own structure, such as Mg2Si or Fe3C. On a phase diagram it shows up as a vertical line at one fixed composition, a distinct phase all of its own, quite unlike the parent metals.
More precisely, an intermetallic (or intermediate) compound is a phase with a definite, narrow composition, a fixed stoichiometry like AxBy, and an ordered crystal structure different from either parent. It appears at a specific composition, often as a near-vertical line, and effectively splits a big diagram into two simpler sub-diagrams. Cementite, Fe3C at 25 atomic percent (6.7 wt%) carbon, is such an intermediate compound, and it dominates the carbon-rich side of the iron-carbon system.
Intermetallics are usually hard and brittle, because their ordered bonding resists dislocation motion. That makes them excellent strengtheners when dispersed as fine precipitates (Ni3Al keeps nickel superalloys strong at high temperature), but troublesome in bulk, where they form brittle joints in solder or weak layers at interfaces. Intermediate phase is the broader term (it can span a small composition range), while compound implies a fixed stoichiometry.
Cementite Fe3C in steel, Mg2Si in aluminum alloys, and Ni3Al in superalloys are all intermetallics.
Fine intermetallic particles strengthen; continuous intermetallic layers embrittle.
An intermetallic is a distinct phase, not a solid solution; it has its own ordered structure, often melts at its own congruent temperature, and is usually hard and brittle.