Pacemaker is an open source high-availability cluster software developed from a collaboration between Red Hat and Novell.
I/O (input/output) fencing is the isolation of a malfunctioning node from a cluster's shared storage to protect the integrity of data.
STONITH (Shoot The Other Node In The Head) is a Linux service for maintaining the integrity of nodes in a high-availability (HA) cluster.
A stretched cluster is a deployment model in which two or more virtualization host servers are part of the same logical cluster but located in separate geographical locations.
A virtual machine (VM) is a software computer that, like a physical computer, runs an operating system and applications.