In general, reliability (systemic def.) is the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances.
 
The IEEE defines it as ". . . the ability of a system or component to perform its required functions under stated conditions for a specified period of time."
 
Reliability may refer to:
 
Reliability (statistics), of a set of data and experiments 
High reliability is informally reported in "nines", 
Data reliability, a property of some disk arrays in computer storage. 
Reliability engineering ensures a system will be reliable when operated in a specified manner 
Reliability theory, as a theoretical concept, to explain biological aging and species longevity 
Reliability (computer networking) is a category used to describe protocols. 
High availability or uptime is the reliability of physical telecommunication networks, computers, and many other types of equipment