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RAMS

Reliability, Availability, Maintainability and Safety

The term "RAMS", originally "RAM" without "S", comes from defense industry and is still today used only in this industry, however, European railway industry has adopted the term "RAMS" in its EN 50126 standard.
RAMS means

Reliability (= item will survive a mission without failure)
Availability (= item will be in operable condition, only little downtime)
Maintainability (= item will be easy to repair and maintain)
Safety (= item will be safe during operation)

The "S" in "RAMS" came decades after "RAM" has been established. However, the "S" makes absolutely sense because the analytical methods for demonstrating "RAM" and "S" are the same.
The following table describes RAMS in more detail.

Term	Intention	Remarks
Reliability	Item must operate with as little failures as possible	There are no requirements for repair time. However, it is important that repairs occur as rarely as possible.
Availability	Item must be in operable condition in as much time as possible	Downtime must be as low as possible. Failures are allowed, provided that repairs take only little time.
Maintainability	Item must be repair friendly	Item design must support quick failure detection and short repair times. This will in turn support high availability.
Safety	Item must enter a safe state upon failure	Product may be unreliable and may have long downtimes and poor repair procedures, provided that, if it fails, it will enter a safe state.

Since the early 1960s, many standards and handbooks have been published by the US department of defense (Mil-HDBK-xxxx and MIL-STD-xxxx).
These documents contain requirements for army material suppliers. In the meantime, almost all of these documents have been withdrawn and made publicly available.
While de jure obsolete, many of them are de facto still authoritative in military and even civil industries.
In contrast to standards and procedures of many other industries at that time, these military requirements not only addressed product quality, but also process and production quality.

The understanding that product quality is rather the result of a good design process than an end test, was traditionally limited to those industries with high safety requirements (aviation, railway, energy supply)

While the focus of RAMS is not primarily safety, it was the intention of combining similar disciplines (RAM and S) that led to the term RAMS.
Despite the differences between RAM and Safety, the analytical methods behind are the same.
According to my experience, the requirement levels can be described as follows:

Goal	Military Products	Industrial Products	Final consumer Products
Reliability	high	medium	low
Availability	high	high	medium
Maintainability	high	medium	low
Safety	high	high	high

Analytical methods for RAMS requirements are described here.