authoritarian vs tyrannical

authoritarian is a noun but tyrannical is not a noun.

Word	Noun	Adjective	Verb	Adverb
authoritarian	Yes	Yes	No	No
tyrannical	No	Yes	No	No

As adjectives, authoritarian and tyrannical are synonyms defined as:

authoritarian and tyrannical: characteristic of an absolute ruler or absolute rule; having absolute sovereignty

Other synonyms of authoritarian include autocratic, despotic, dictatorial, tyrannic.

authoritarian (noun)	tyrannical (noun)
a person who behaves in a tyrannical manner

authoritarian (adjective)	tyrannical (adjective)
expecting unquestioning obedience	marked by unjust severity or arbitrary behavior
characteristic of an absolute ruler or absolute rule; having absolute sovereignty	characteristic of an absolute ruler or absolute rule; having absolute sovereignty