Recovery - a key feature of a high availability system, this is the mechanism by which a system which is experiencing system failures restarts those services. A system may need to start a recovery process for a number of reasons - e.g. deadlock, priority inversion, livelock, resource exhaustion. Often recovery is achieved by the hardware watch dog which reboots the system.
Regression Test - re-running a test to verify that features previously verified still work after bug fixes or feature addition - intended to prevent unintentional interactions between software modifications that might introduce new problems.
Relay - a mechanical or solid state device which provides a simple switch - e.g. double pole double throw or single pole single throw.
Reliable Transport - a data transport protocol on a network which includes error detection/correction, retransmission and supports diverse routing such that overall data is delivered if at all possible.
Resource Arbiter - a sub-system which implements a resource grant policy - e.g. a a bus arbiter coordinates bus grants for bus requests from multiple masters and targets.