dunno the exact definition.
I will give an example i hope dat i din gave wrong one.
Suppose we have two sets
Set A has Sachin,Kohli,Sehwag
Set B has 50 s,100s,150s in ODIs
Now i can mapp Sachin to 50,100,150
And kohli to 100,50
And sehwag to 50,100,150
now we have many many mapping
dats we have more than one images for elements of set A,and preimages.