Reliability is:

• The ability of a product or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances
• The idea that something is fit for purpose with respect to time
• The capacity of a device or system to perform as designed
• The resistance to failure of a device or system
• High availability of a product
• High maintainability of a product
• A high probability of mission success
• Has to do with the quality of measurement
• Is the correlation of an item, scale, or instrument with a hypothetical one which truly measures what it is supposed to
• The trait of being dependable or reliable - worthy of reliance or trust
• A statistical process

• A quality of a measurement indicating the degree to which the measure is consistent, that is, over repeated measurements would give the same result
• Something that is reliable, can be trusted or believed because it works or behaves well in the way you expect
• The quality of a product