Geez ... what's the difference between a bicycle and a unicycle? One has two wheels, one has one wheel. Bi = 2 and Uni = 1. Therefore, a bidirectional relationship must have two directions, easily labeled 'to' and 'from''. A unidirectional relationship likewise has one direction.
ONE - ONE Bidirectional ? < ------ > ONE - ONE Unidirectional ? --------->
Here is my understanding: Suppose there are two beans A and B. Bidirectional means: A can get B, and B can get A too; In A: getB(); In B: getA(); Unidirectional means: A can get B, but B can't get A. In A: getB(); In B: no method to get A.