Reliability is an attribute of any computer-related component (software, or hardware, or a network, for example) that consistently performs according to its specifications.
It has long been considered one of three related attributes that must be considered when making, buying, or using a computer product or component.
Reliability, availability, and serviceability - RAS, for short - are considered to be important aspects to design into any system.
In theory, a reliable product is totally free of technical errors; in practice, however, vendors frequently express a product's reliability quotient as a percentage.
Evolutionary products (those that have evolved through numerous versions over a significant period of time) are usually considered to become increasingly reliable, since it is assumed that bugs have been eliminated in earlier releases.