The softening point of a material (that does not have a specific melting point) is the thermodynamic state during which the material starts changing its state of aggregation, from solid to fluid.

Contrary to a commonly-held belief, glass is not a solid but an under-cooled fluid.

Its softening point corresponds to specific temperature and pressure conditions: under these conditions the glass collapses upon itself

Each glass, according to its features, is marked by a different softening point.