Refers to oppressive, cruel government or rule. Often used in historical or political contexts.
That country has suffered under tyranny for many years.
The history textbook contains stories of kings who ruled with tyranny.
Compound of 虐 (ぎゃく, 'oppress, tyrannize') and 政 (せい, 'government, rule'). The exact historical derivation is uncertain, but the meaning is transparent from the kanji.