Bilateral element: The element in which the voltage current relationship is same for current flowing in either direction( means changing input and output terminal no impact on the operation), is known as bilateral element. Example- transformer, resistor, inductance, capacitance etc.
Unilateral element: The element in which the voltage current relationship is not same for current flowing in either direction (means changing input and output terminal impact on the operation) is known as unilateral element. Example- vaccum tubes, diodes, transistors etc.