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Minimum Cardinality in an ER diagram represents the minimum number of occurrences of an entity that can be associated with a related entity. It is denoted by a small line or tick mark on the relationship line between two entities. For example, a customer can have a minimum cardinality of 1 with an order because every order must have at least one customer associated with it. Similarly, a product can have a minimum cardinality of 0 with an order because an order can exist without any products associated with it.