继电器是一种电控制器件，是当输入量（激励量）的变化达到规定要求时，在电气输出电路中使被控量发生预定的阶跃变化的一种电器。
Relay is a kind of electrical control device. When the change of input (excitation) reaches the required requirements, it makes the controlled quantity change step by step in the electrical output circuit.
它具有控制系统（又称输入回路）和被控制系统（又称输出回路）之间的互动关系。通常应用于自动化的控制电路中，它实际上是用小电流去控制大电流运作的一种“自动开关”。故在电路中起着自动调节、安全保护、转换电路等作用。
It has the interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). Usually used in automatic control circuit, it is actually a kind of "automatic switch" which uses small current to control the operation of large current. Therefore, it plays the role of automatic regulation, safety protection, conversion circuit and so on.