  A novel way of characterizing the process of reaching consensus for a social
system is given. The foundation of the characterization is based on the theorem
which states that the sufficient and necessary condition for a system to reach
the state of consensus is the occurrence of communicators, defined as the units
of the system that can directly communicate with all the others simultaneously.
A model is proposed to illustrate the characterization explicitly. The
existence of communicators provides an efficient way for unifying two systems
that a state of consensus is guaranteed after the mergence.
