"The average fraction of all the rows in an index that have the same key value. Density is 1 if all of the data values are the same and 1/N if every data value is unique." (Karen Paulsell et al, "Sybase SQL Server: Performance and Tuning Guide", 1996)
"The relative percentage of a multidimensional structure's cells that contain data. OLAP Services stores only cells that contain data. A dense cube requires more storage than a sparse cube of identical structure design." (Microsoft Corporation, "SQL Server 7.0 System Administration Training Kit", 1999)
"The reciprocal of the count of the number of distinct keys. For example, suppose you have these key values: {A,C,C,D,D}. The number of distinct keys, that is, the number that a SELECT DISTINCT ... statement would return, is three: {A,C,D}. Now recall from your arithmetic classes that reciprocal = 1/N—that is, the reciprocal of a number N is the number that would yield one when multiplied by N. The reciprocal of 3 is 1/3; therefore, the density for our example is 1/3. It's possible to get a preliminary guess of the number of rows that column1 = <literal> will return by multiplying the table's cardinality by the density of the index." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)
"The degree of duplicate values for the data contained in a table column." (Victor Isakov et al, "MCITP Administrator: Microsoft SQL Server 2005 Optimization and Maintenance (70-444) Study Guide", 2007)
"A decimal number between 0 and 1 that measures the selectivity of a column. Values close to 1 indicate that the column is unselective, whereas values close to 0 indicate that this column is more selective." (Oracle, "Database SQL Tuning Guide Glossary", 2013)
"1. In an index, the frequency of duplicate values. 2. In a data file, a percentage that indicates how full a data page is. 3. In Analysis Services, the percentage of cells that contain data in a multidimensional structure." (Microsoft Technet
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