[index covering]"A data access condition where the leaf-level pages of a nonclustered index contain the data needed to satisfy a query. The index must contain all columns in the select list as well as the columns in the query clauses, if any. The server can satisfy the query using only the leaf level of the index. When an index covers a query, the server does not access the data pages." (Karen Paulsell et al, "Sybase SQL Server: Performance and Tuning Guide", 1996)
"A nonclustered index that contains all of the columns required to satisfy a query, in both the selection list and the WHERE clause." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)
"An index that contains every column in the select list of a query." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)
"An index where the values of the data are stored in the index, allowing data retrieval from the index itself, instead of the data object." (DAMA International, "The DAMA Dictionary of Data Management", 2011)
