  First order phase transitions in finite systems can be defined through the
bimodality of the distribution of the order parameter. This definition is
equivalent to the one based on the inverted curvature of the thermodynamic
potential. Moreover we show that it is in a one to one correspondence with the
Yang Lee theorem in the thermodynamic limit. Bimodality is a necessary and
sufficient condition for zeroes of the partition sum in the control intensive
variable complex plane to be distributed on a line perpendicular to the real
axis with a uniform density, scaling like the number of particles.
