  The present work analyzes the meaning of the Weak Equivalence Principle in
the context of quantum mechanics. A quantal definition for this principle is
introduced. This definition does not require the concept of trajectory and
relies upon the phase shift induced by a gravitational field in the context of
a quantum interference experiment of two coherent beams of particles. In other
words, it resorts to wave properties of the system and not to classical
concepts as the idea of trajectory.
