"A database object that consists of key values from the data tables, and pointers to the pages that contain those values. Indexes speed up access to data rows." (Karen Paulsell et al, "Sybase SQL Server: Performance and Tuning Guide", 1996)
"A set of pointers that are logically ordered by the values of a key. An index is a database object that provides access to data in the rows of a table, based on key values. Indexes provide quick access to data and can enforce uniqueness on the rows in a table. SQL Server supports clustered and nonclustered indexes. In a clustered index, data is stored in the same order as the index; in a nonclustered index, data is stored differently from the index." (Patrick Dalton, "Microsoft SQL Server Black Book", 1997)
"A database object that provides fast access to data in the rows of a table, based on key values. Indexes can also enforce uniqueness on the rows in a table. SQL Server supports clustered and nonclustered indexes." (Anthony Sequeira & Brian Alderman, "The SQL Server 2000 Book", 2003)
"A database object designed to reduce the amount of time it takes to retrieve rows from a table. An index is created based on one or more columns in the table." (Bob Bryla, "Oracle Database Foundations", 2004)
"In a relational database, a database object that provides fast access to data in the rows of a table, based on key values. Indexes can also enforce uniqueness on the rows in a table. SQL Server supports clustered and non-clustered indexes. The primary key/unique constraint automatically causes an index to be built. In full-text searches, a full-text index stores information about significant words and their location within a given column." (Thomas Moore, "EXAM CRAM™ 2: Designing and Implementing Databases with SQL Server 2000 Enterprise Edition", 2005)
"A data structure that uses one or more columns to make looking up values on those columns faster." (Rod Stephens, "Beginning Database Design Solutions", 2008)
"In a relational database, a database object that provides fast access to data in the rows of a table, based on key values. The primary key of a table is automatically indexed." (Jim Joseph, "Microsoft SQL Server 2008 Reporting Services Unleashed", 2009)
"A physical database object used to quickly access data in a database." (Craig S Mullins, "Database Administration", 2012)
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