In relational database design, the closure of an attribute set is a crucial concept used to determine all possible attributes that can be functionally determined from a given set of attributes using known functional dependencies. The closure helps database designers check if an attribute set is a candidate key and plays a vital role in database normalization.

The closure of an attribute set X, denoted as X+, is the set of all attributes that can be functionally determined by X based on a given set of functional dependencies.