Dictionary:

tyrannical

  (tĭ-răn'ĭ-kəl, tī-) also tyrannic (-răn'ĭk)

1. Of or relating to a tyrant or tyranny: a tyrannical government.
2. Characteristic of a tyrant or tyranny; despotic and oppressive: a tyrannical supervisor.

adjective

1. Characterized by or favoring absolute obedience to authority: authoritarian, autocratic, despotic, dictatorial, totalitarian, tyrannic. See over/under.
2. Having and exercising complete political power and control: absolute, absolutistic, arbitrary, autarchic, autarchical, autocratic, autocratical, despotic, dictatorial, monocratic, totalitarian, tyrannic, tyrannous. See over/under, politics.

Definition: dictatorial
Antonyms: democratic, easy-going, liberal

The adjective has 3 meanings:

Meaning #1: of or relating to or associated with or resembling a dictatorship
  Synonym: tyrannic
  Pertains to noun: tyranny (meaning #1)

Meaning #2: marked by unjust severity or arbitrary behavior
  Synonyms: oppressive, tyrannous

Meaning #3: characteristic of an absolute ruler or absolute rule; having absolute sovereignty
  Synonyms: authoritarian, autocratic, dictatorial, despotic