  Amorphous solids yield at a critical value of the strain (in strain
controlled experiments); for larger strains the average stress can no longer
increase - the system displays an elasto-plastic steady state. A long standing
riddle in the materials community is what is the difference between the
microscopic states of the material before and after yield. Explanations in the
literature are material specific, but the universality of the phenomenon begs a
universal answer. We argue here that there is no fundamental difference in the
states of matter before and after yield, but the yield is a bona-fide first
order phase transition between a highly restricted set of possible
configurations residing in a small region of phase space to a vastly rich set
of configurations which include many marginally stable ones. To show this we
employ an order parameter of universal applicability, independent of the
microscopic interactions, that is successful in quantifying the transition in
an unambiguous manner.
