A type of index mostly used in SQL-Server, which aligns the data of a table with the index.
CLUSTERED INDEX is used to organize an entire table in a specified order. The data pages in the clustered index are the pages of the table - the index itself contains all the data.
A helpful analogy is a phone book - the pages are in the order of
LastName, Firstname but contain all the data about individual entries as well.
Similar concepts exist in other database systems, such as
INDEX ORDERED TABLES in oracle.