Bijection

A bijection is a mapping between two sets of equal cardinality and each member of the first set maps to a unique element in the second set. Because this is true, it is also true that each element in the second set maps to a unique element in the first set.

example

A={1,2,3}

B={2,4,6}

f(x)=2x maps from A to B.

f(x)=x/2 maps from B to A.