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In chemistry, materials science, and physics, the 'solidus' is a line on a phase diagram below which a given substance is stable in the solid phase. Most commonly, this line represents a transition temperature. The solidus may be a straight line, or it may be curved, depending upon the substance. The solidus is most often applied to binary systems such as solid solutions, including metal alloys.
The solidus may be contrasted to the liquidus. The solidus and liquidus do not necessarily align or overlap. If a gap exists between the solidus and liquidus, then within that gap, the substance is not stable as either a solid or a liquid; such is the case, for example, with the olivine (forsterite-fayalite) system.
Note that water is very unusual in having a solidus temperature that decreases with increasing pressure. Most materials that do not contain water are more prone to solidifying (higher solidus temperature) under higher pressure.