How consistently a thing does what it should. Reliability is not correctness — a thing can fail dependably.
Built on rely, from Old French relier (to bind again), from Latin religare: re- (again) + ligare (to bind); reliability is a measure of how well a thing stays bound to what it is supposed to do, whether or not it does that thing correctly.
How consistently a thing does what it should. The root's binding is a separate claim from correctness: reliability describes a thing bound to a behaviour, even when that behaviour is a failure it delivers dependably.