For other uses: see solidus (disambiguation).

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.