• System – A composite, at any level of complexity, of operational and support equipment, personnel, facilities and software which are used together as an entity and capable of performing and supporting an operation role.
• System Effectiveness – is the probability that the system can successfully meet an operational demand within a given time when operated under specified conditions.
• System Effectiveness – (for a oneshot device such as a missile) is the probability that the system (missile) will operate successfully (kill the target) when called upon to do so under specified conditions.
• Reliability – is the probability that an item will perform its intended function for a specified interval under stated conditions.
• Mission Reliability – is the probability that an item will perform its intended function for a specified interval under stated conditions. is the ability of an item to perform its required functions for the duration of a specified mission profile.
• Operational Readiness – is the ability of an item (military unit) to respond to its operational plan(s) upon receipt of an operating order (total calendar time is the basis for computation of operational readiness).
• Operational Readiness – is the ability of an item to respond to its operational plan(s) upon receipt of an operating order (total calendar time is the basis for computation of operational readiness).
• Availability – is a measure of the degree to which an item is in an operable and committable state at the start of a mission when the mission is called for at an unknown (random) time (includes operating time, active repair time, adminstrative time, and logistic time, but excludes mission time).
• Intrinsic Availability – is the probability that the system is operating satisfactorily at any point in time when used under stated conditions, where the time considered is operating time and active repair time.
• Design Adequacy – is the probability that the system will successfully accomplish its mission, given that the system is operating within design specifications.
• Maintainability – is the measure of the ability of an item to be retained in, or restored to, specified condition when maintenance is performed by personnel having specified skill levels, using prescribed procedures and resources, at each prescribed level of maintenance and repair.
• Serviceability – is the probability that a failed system will be restored to operable condition within a specified active repair time.
• Continuity of service – is the ability of a system to provide service over a specified period of time without interruption.  Continuity of service is also referred to as reliability.

References:

1. MIL-HDBK-338, Electronic Reliability Design Handbook, 15 Oct 84
2. MIL-STD-721, Definitions of Terms for Reliability and Maintainability, 23 Oct 91