Cardinality
Two sets have the same (or equal) cardinality if and only if they have the same number of elements, which is the another way of saying that there is a 1-to-1 correspondence between the two sets. The cardinality of the set A is less than or equal to the cardinality of set B if and only if there is an injective function from A to B. The cardinality of the set B is greater than or equal to the cardinality of set A if and only if there is an injective function from A to B.