Definition of irrigation. 1  1 : the therapeutic flushing of a body part with a stream of liquid. 2  2 : the watering of land by artificial means to foster plant growth.
Answer: To start with, Oracle Real Application Clusters (RAC) are a database system involving a configuration of multiple hosts or servers joined together with clustering software and accessing the shared disk storage structures.
From Oracle FAQ. The Oracle System ID (SID) is used to uniquely identify a particular database on a system. For this reason, one cannot have more than one database with the same SID on a computer system. When using RAC, all instances belonging to the same database must have unique SID's.