<firstterm>cluster</firstterm>.
     A database cluster is created using the
     <xref linkend="app-initdb" /> program.
    </para>
    <para>
     In <productname>PostgreSQL</productname>, the term
     <firstterm>cluster</firstterm> is also sometimes used to refer to an instance.
     (Don't confuse this term with the SQL command <command>CLUSTER</command>.)
    </para>
    <para>
     See also <glossterm linkend="glossary-cluster-owner">cluster owner</glossterm>,
     the operating-system owner of a cluster,
     and <glossterm linkend="glossary-bootstrap-superuser">bootstrap superuser</glossterm>,
     the <productname>PostgreSQL</productname> owner of a cluster.
    </para>
   </glossdef>
  </glossentry>

  <glossentry>
   <glossterm>Database server</glossterm>
   <glosssee otherterm="glossary-instance" />
  </glossentry>

  <glossentry id="glossary-database-superuser">
   <glossterm>Database superuser</glossterm>
   <glossdef>
    <para>
     A role having <firstterm>superuser status</firstterm>
     (see <xref linkend="role-attributes"/>).
    </para>
    <para>
     Frequently referred to as <firstterm>superuser</firstterm>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-data-directory">
   <glossterm>Data directory</glossterm>
   <glossdef>
    <para>
     The base directory on the file system of a
     <glossterm linkend="glossary-server">server</glossterm> that contains all
     data files and subdirectories associated with a
     <glossterm linkend="glossary-db-cluster">database cluster</glossterm>
     (with the exception of
     <glossterm linkend="glossary-tablespace">tablespaces</glossterm>,
     and optionally <glossterm linkend="glossary-wal">WAL</glossterm>).
     The environment variable <literal>PGDATA</literal> is commonly used to
     refer to the data directory.
    </para>
    <para>
     A <glossterm linkend="glossary-db-cluster">cluster</glossterm>'s storage
     space comprises the data directory plus any additional tablespaces.
    </para>
    <para>
     For more information, see
     <xref linkend="storage-file-layout"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-data-page">
   <glossterm>Data page</glossterm>
   <glossdef>
    <para>
     The basic structure used to store relation data.
     All pages are of the same size.
     Data pages are typically stored on disk, each in a specific file,
     and can be read to <glossterm linkend="glossary-shared-memory">shared buffers</glossterm>
     where they can be modified, becoming
     <firstterm>dirty</firstterm>.  They become clean when written
     to disk.  New pages, which initially exist in memory only, are also
     dirty until written.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-datum">
   <glossterm>Datum</glossterm>
   <glossdef>
    <para>
     The internal representation of one value of an <acronym>SQL</acronym>
     data type.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-delete">
   <glossterm>Delete</glossterm>
   <glossdef>
    <para>
     An <acronym>SQL</acronym> command which removes
     <glossterm linkend="glossary-tuple">rows</glossterm> from a given
     <glossterm linkend="glossary-table">table</glossterm>
     or <glossterm linkend="glossary-relation">relation</glossterm>.
    </para>
    <para>
     For more information, see
     <xref linkend="sql-delete"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-domain">
   <glossterm>Domain</glossterm>
   <glossdef>
    <para>
     A user-defined data type that is based on another underlying data type.
     It acts the same as the underlying type except for possibly restricting
     the set of allowed values.
    </para>
    <para>
     For more information, see <xref linkend="domains"/>.
    </para>
   </glossdef>
  </glossentry>

  <glossentry id="glossary-durability">
   <glossterm>Durability</glossterm>
   <glossdef>
    <para>
     The assurance that