Solidus
Index
The solidus lines on a phase diagram represent the locus of all points that represent the completion of the solidification of a material of given composition as it is cooled in equilibrium from the liquid phase. For the copper-silver binary phase diagram shown two such lines exist, one for the 
(a + L) two phase zone and the other for the (b + L) zone.
From: Callister, 
"Materials Science and Engineering," 
Wiley (1994)