What is the term for an individual who speaks out against the regime in power?

The term that best describes an individual who speaks out against the regime in power is "dissident." Dissidents often oppose the policies or actions of the government, commonly advocating for reform, change, or even the overthrow of the current system. This engagement typically implies a risk, as dissent can lead to backlash from authorities.

Understanding the distinctions among the other choices clarifies why "dissident" is the correct answer. An activist is someone who actively campaigns for a cause or policy change but may not always be opposing a specific regime. A revolutionary is usually involved in advocating for a significant change at the systemic level, possibly involving the overthrow of a government, but does not specifically describe someone who simply speaks out. An opposition leader is a more formal title typically assigned to a recognized figure in a political context who leads a party or movement against the ruling authority, which may not capture the broader implications of dissent against any regime.