Is a diode active or passive?

Hint: To answer this question we need to first understand the difference between active elements and passive elements. Electronic components can be both active and passive elements. These electric components are resistors, inductors, diodes, etc. The main difference between these two is simpler words: active elements have the ability to produce energy whereas passive elements cannot supply any energy.

Complete answer:
There are different opinions on the functioning of diodes since the question does not provide information about what type of diode it is. Therefore we can consider the simple rectifier diode. The rectifier diode is a conventional diode which is also called a p-n junction diode that has anode and cathode as two terminals. The current-voltage characteristics of the p-n junction diode are non-linear and monotonic and do not show a negative differential resistance. This diode allows current flow in one direction and will not allow in another direction. Simply it acts as a switch and it can to some extent dissipate electric energy. This is a function of passive components. And therefore diodes are a passive component.
Therefore a diode is a passive component.

Note:
However, all diodes are not passive electrical components. For example, there are microwave diodes such as the Gunn diode and the Impatt diode. These diodes have a negative differential resistance. Having negative differential resistance means that the diode will act as an active electrical component. Therefore these diodes which have negative differential resistance are active electrical components.