Source : Free On-Line Dictionary of Computing

Church-Rosser Theorem
     
        This property of a {reduction} system states that if an
        expression can be reduced by zero or more reduction steps to
        either expression M or expression N then there exists some
        other expression to which both M and N can be reduced.  This
        implies that there is a unique {normal form} for any
        expression since M and N cannot be different normal forms
        because the theorem says they can be reduced to some other
        expression and normal forms are irreducible by definition.  It
        does not imply that a normal form is reachable, only that if
        reduction terminates it will reach a unique normal form.
     
        (1995-01-25)