What is Reliability?

The dictionary’s definition of reliability as a noun is the state of being reliable and
something that can be relied upon or is dependable. British Standard BS 4778 defines reliability as the ability of an item to perform a required function under stated conditions for a stated period of time. In a similar way ISO 8042 defines it as the ability of an item to perform a required
function under given environmental and operational conditions and for a stated period
of time.