What is cardinality in data modeling? And how do you implement cardinality in databases? This discussion uses simple, easy-to-follow examples to describe both the theory and modeling of cardinality in ER diagrams. Cardinality is a mathematical term. It translates into the number of elements in a set. In databases, cardinality refers to the relationships between the data in two database tables. Cardinality defines how many instances of one entity are related to instances of another entity.