noun
a government agency responsible for the protection of a country's leaders and the investigation of espionage, sabotage, and other threats to national security
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The secret service is responsible for protecting the President of the United States.
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I heard the secret service is always on high alert.
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The secret service guys are like ninjas, always watching.
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In a way, our immune system can be seen as the body's secret service, protecting us from harmful invaders.