The RAC means real application clusters in Oracle. Oracle real application clusters is an option to the award-winning Oracle Database Enterprise Edition. Oracle RAC is a cluster database with a shared cache architecture that overcomes the limitations of traditional shared-nothing and shared-disk approaches to provide highly scalable and available database solutions for all your business applications. Oracle RAC is a key component of Oracle's private cloud architecture. Oracle RAC support is included in the Oracle Database Standard Edition for higher levels of system uptime.
 
Oracle RAC One Node is a new option available with Oracle Database 11g Release 2. Oracle RAC One Node is a single instance of an Oracle RAC enabled database running on one node in a cluster. This option adds to the flexibility that Oracle offers for database consolidation while reducing management overhead by providing a standard deployment for Oracle Databases in the enterprise.

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http://www.oracle.com/technetwork/database/clustering/overview/index.html