1. The capacity of being dependable or trustworthy: Over all the years, the old car certainly showed total reliability in that it always started in the morning, even in the winter months.
2. The tendency of a measuring system, test, or observation to be resistant to failure: In the past years, studies have shown that there has been no reliability of it snowing around Christmas time, like it did a decade ago.

The amount of power necessary for a transmission to make certain that its network will be secure under a reasonable range of uncertainties in system conditions: Jim learned about the transmission reliability margin in his computer class and realized how important it was.