What is closed frequent itemset in datamining?

An itemset is closed if none of its immediate supersets has the same support as the itemset.

So for example, if {Bread, Milk} is an itemset that has support=4, and all of its supersets has support<4, then {Bread, Milk} is a closed itemset.

Counter e.g.: If, let's say, {Bread, Milk, Sugar} has support=4, then {Bread, Milk} is not a closed itemset anymore.

Note: The definition states "the same" and doesn't say "the same or more" because it's impossible for a superset to have a support greater than one of its subsets.