The Edit of Milan was a proclamation, initiated by Constantine in the West, but agreed to by Licinius in the East, by which Christianity was given legal status, equal to paganism if not a little superior to it. Persecution in any form from 313 was supposed to come to a stop.
For more on this, see the following:
%between%and in the Catholic Encyclopedia:
Constantine the Great
Fr. Vincent Serpa, O.P.