1350–1400;Middle English < Late Latinsolidus (nummus) a solid (coin), a gold (coin)
(on a graph of temperature versus composition) the curve connecting the temperatures at which a solid solution is in equilibrium with its vapor and with the liquid solution, and therefore connecting melting temperatures of solid solutions.
late 14c., plural solidi, used of both English shilling and Roman gold coin, from Late Latin solidus, an imperial Roman coin (worth about 25 denarii), from nummus solidus, literally "solid coin" (see solid (adj.)).
The maximum temperature at which all components of a mixture (such as an alloy) can be in a solid state. Above the solidus some or all of the mixture will be in a liquid state. See illustration at eutectic. Compare liquidus.