What is a unilateral element ?

Suppose, an electrical element is connected with a voltage source. If the element is unilateral, then whenever there is a change is occurred in the polarity of the applied voltage, that is the positive & negative terminal of the source altered, the magnitude of the current passing through the element is affected by the polarity change.

Think about a simple diode, which is connected with a voltage source. If somehow the polarity is changed, that is the (+) & (−) terminal interchanged, then the diode will not work. So, we can say, diode is a unilateral element. An important feature of a unilateral element is, it offer varying impedances with variations in flow of current.

Basically, semiconductor devices like diodes, transistors, operational amplifiers are the unilateral element.