What is quorum?

In computers :

Quorum refers to the minimum numbers of nodes within the peer domain that are required to carry out a particular operation. There are three kinds of quorum that specify the number of nodes required for different types of operations.

Taken from IBM, RSCT Administration Guide

URL :

http://publib.boulder.ibm.com/infocenter/clresctr/vxrx/index.jsp?topic=/com.ibm.cluster.rsct_csm16.admin.doc/bl503_wiquor.html