"The probability that an item will perform a required function without failure under stated conditions for a stated period of time."
Design objectives that refers to an item's reliability.
"As concerns the gold ring set with an Emerald, we guarantee that for 20 years the emerald will not fall out of the gold ring. If the emerald should fall out of the gold ring before the end of 20 years, we shall pay unto Bel-Nadin-Shumu and indemnity of 10 mana of silver."
Record found on a clay tablet in Egypt - 429. BC
Notice that except for probability, all of the element in the definition of reliability are found.
Probability methods are used to determine system reliability using knowledge of system architecture and part reliability.
Statistical methods are used to determine part reliability using part failure data (reliability data). A part is an item that is assumed to be indivisible for the purpose of determining its reliability
Repairable System Reliability
Use of reliability figures of merit that require some knowledge of system repair activity.
Non-Repairable System Reliability
Use of reliability figures of merit that do not require knowledge of repair activity (if any)