单一故障是指一种导致系统或者系统组件在执行其预期安全功能时失效的故障，和任何随之引发的故障。（a failure which results in the loss of capability of a system or component to perform its intended safety function(s),and any consequential failure(s) which result from it.（IAEA Safety Glossary,Ed.2.0,2006））

单一故障（Single Failure）

单一性的范围：由定义可以知道，单一故障的单一性体现在安全功能方面，也就是说单一故障是从功能角度划分的。

单一故障状态（Single Failure Condition）（IEC 60601-1）

降低风险的一种方式存在缺陷或发生了一个故障时的状态。

Condition in which a single means for reducing a RISK is defective or a single abnormal condition is present

单一故障安全（Single Failure Safe）（IEC 60601-1）

医用电气设备或其部件，在单一故障条件下，预期的生命周期内发生不可接受的风险是可忽略时的特性。

Characteristic of ME EQUIPMENT or its parts whereby it remains free of unacceptable risk during its EXPECTED SERVICE LIFE under Single Failure Conditions