What is a clustered index?

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"A clustered index is an index which defines the physical order in which the table of records is organized in a database. Since records are physically stored in a database table in only one way, there is one clustered index per table. A clustered index is created on a primary key column by default. Clustered indexes differ from non-clustered indexes because a non-clustered index is a special type of index in which the logical order does not match the physical order of the rows in the table."