The term "iron tyranny" refers to

The term iron tyranny refers to the arbitrary and unrestrained use of the power vested by the people.

Explanation:

Iron tyranny was a term that was used by John F Kennedy in the inaugural address of his Presidency to deride the opponents and their wish to overpower the sentiment of the people and to also contrast himself with them by making that comparison and by the virtue of it too.

The comparison that was drawn here was that the powers opposite wanted to have arbitrary and unrestrained access to the power vested by the people to the President.

The address worked and conveyed the message effectively.