A general definition is "oppressive power."

A tyranny must be recognized. See: https://en.wikipedia.org/wiki/American_Revolution

"The Continental Congress declared King George a tyrant who trampled the colonists' rights as Englishmen..."

If you recognize a tyranny but do nothing, you stand to receive further oppression. Consider the case of the PRC. The CCP is tyrannical toward those they rule; we plainly see this reality from our perch of liberty. However, unless the Chinese people recognize the tyrannical nature of the CCP, they'll continue receiving oppression. Making this realization harder is the PRC's surveillance apparatus and the population's indoctrination. I wish them luck.