Delusion or Overvalued idea?

What is a delusion?
I am talking about strictly psychiatry. As a kind of thought disorder. Not in our common everyday sense of ' I think he has a delusion that he is very important' and so and so.
In psychiatry delusion is defined as ' a firm abnormal unshakable belief held with adequate conviction which is not shared or derived from his socio cultural background'.
Someone saying that aliens inserted a machine in his head is having a delusion.(until X Files reveal with real proof that indeed its happening)
The bold texting is mine. If you remove that clause every one will be having a delusion. But even believers say 'I believe in god', not 'I know there is god'. For that matter atheists also. So I guess deep down its shakable.
There is another concept called 'overvalued idea'. Here the belief in itself is not abnormal but the behaviour related to it is abnormal. The person become so much preoccupied with his belief that it gains precedence over all his daily activities.
A person who says 'we should plant trees for our environment' is not talking abnormally. But when he starts spending all his time and money on this avoiding his family and business it becomes overvalued idea. It in itself is not diagnostic of any psychiatric condition.

Now tell me something about all those people who get into some train to mumbai to become a movie star but has never acted in their life except in front of their mirrors. Or actresses who live on mineral water and vegetables to reach the 'size 0' through out their healthy digestible adult life. Or people who spend all their hard earned bank balance and future credit on buying 'brand'y apparels and expensive cars hoping that it will make up for the ailment of a lack of meaningful respected existence. Or someone ready to kill for money to get a decent booze even if it means that after a day what will be left is a hangover, a bloody knife and an entire life inside four walls . Or all those aspiring professionals who burn their midnight oil hoping for a secure future while world is blooming, withering and preparing for rebirth outside their life proof walls. Or all those who are wallowing in the exhilarating intoxication of hatred towards a country, caste or creed forgetting that the enemy in whose throat your knife is just an accidental, innocent product of a sperm and egg who didn’t even know that they existed let alone the fact that they are going to generate a person with greed and fear and insecurities.

Tell me something about all these people. Are they having a delusion or an overvalued idea?
At least in some cases I am not able to make up my mind.