A patient is fixedly convinced, with no realistic basis, that a specific government agency is monitoring his every move and plotting to harm him, a belief that has been present and unshaken for months despite no supporting evidence whatsoever. What is this?

This is a delusion of persecution: a fixed, false belief of being watched, plotted against, or otherwise deliberately targeted for harm by other people or organisations, here a government agency, held with unshaken conviction over months despite no supporting evidence. It is not a realistic fear grounded in an actual threat, which should always be considered and, where relevant, taken seriously in its own right before assuming a belief is delusional; here, there is explicitly no evidence supporting the belief. It is not delusional perception, which is one specific mechanism by which a persecutory delusion can arise from a normally perceived stimulus, not described in this vignette. And it is not anxious foreboding, a sense of impending misfortune without necessarily any specific persecutory content or fixed belief about particular persecutors.