Now apparently delusion as a technical term in psychiatry has been refined so it is not the belief itself but the *way* it is believed that makes it 'delusional'.
http://www.theguardian.com/science/2013/aug/04/truly-madly-deeply-delusionalDelusions, in the medical sense, are not simply a case of being mistaken, as the everyday use of the term suggests. They are profound and intensely held beliefs that seem barely swayed by evidence to the contrary – even to the point of believing in the bizarre. My heart has been replaced by steam. My thoughts are being stolen by satellites. The government communicates with me through birdsong.
But many delusions are not outlandishly eccentric, they are simply implausible. Consider the scenario where people believe that their neighbours are conspiring against them or that they are the subject of a film star's secret affections. Occasionally, these beliefs turn out to be true, but this is not a reliable guide to whether someone is delusional or not. This was memorably illustrated by the psychiatrist Andrew Sims, who warned in his psychopathology textbook Symptoms in the Mind that spouses of people with delusions of infidelity may occasionally be driven to infidelity. This romantic betrayal does not suddenly cure their partner of their mental illness.
The general idea is that delusions represent a problem with how you believe – that is, a problem with forming and changing beliefs – not a problem with what you believe. In other words, simply believing something strange or unusual should not be considered a problem but having "stuck" beliefs that are completely impervious to reality suggests something is mentally awry.
On the ground, mental health professionals are often required to decide if someone's thinking indicates a disturbance in their understanding of the world, and this is where the new DSM-5 definition of a delusion may usher in a quiet revolution in psychiatry. No longer are psychiatrists asked to decide whether the patient has "a false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary". A wordy and unhelpful definition that has so many logical holes you could drive a herd of unicorns through it.
Instead, the new definition of delusions describes them as fixed beliefs that are unswayed by clear or reasonable contradictory evidence, which are held with great conviction and are likely to share the common themes of psychosis: paranoia, grandiosity, bodily changes and so on. The belief being false is no longer central and this step forward makes it less likely that uncomfortable claims can be dismissed as signs of madness.