機器或其組件或設備，在規定的條件及給定的時間，執行必要的功能不會失效的能力。Ability of a machine or its components or equipment to perform a required function under specified conditions and for a given period of time without failing. (ISO 12100:2010, 3.2)
結構或結構元素履行特定要求的能力，包括設計之工作壽命。Ability of a structure or structural element to fulfill the specified requirements, including the working life, for which it has been designed. (ISO 2394:1998, 2.2.7)
The ability of an item to perform a required function under stated conditions for a specified period of time. NOTE. The term reliability is also used as a reliability characteristic denoting a probability of success, or as success ratio. (ARMP-7:2001)
The probability that an item will performs its intended function for a specified period of time under stated conditions. (MIL-STD-1903C:1983, 3.1.514)
(1) The probability of failure-free performance for a specified interval under stated conditions. (2) The probability that an item can perform its intended function for a specified interval under stated conditions. (For non-redundant items this is equivalent to definition (1). For redundant items this is equivalent to definition of mission reliability). (MIL-STD-1309D:1992)
當進行測驗或檢定時，數據的可靠度，也就是數據可以相信的程度，一般稱為信度，其定義為："The reliability of a test is the consistency with which a test measures any attribute."，相對的指標為效度(validity)，其定義為："The validity of a test is how well it does indeed measure what it purports to measure."