Delusions of grandeur

Delusions of grandeur is a phrase used to describe those who have a highly inflated sense of importance. It is akin to megalomania in the sense that they both relate to the idea of narcissism, except that megalomania is classified as either "a psychological condition or an absolute obsession with having power, wealth, or grandiose things", as opposed to the delusion of actually having them. Grandiose delusions, sometimes called the "expansive personality", are commonly found in the manic phase of bipolar disorder and consist of bizarre inflated beliefs about one's own social status, influence, and abilities (typical themes include being incredibly rich, being a religious figure like Jesus or the Virgin Mary, being royalty, etc.) ; they are typically treated with antipsychotics and other medications.

Informal usage
The term is also used colloquially to refer to hubris, or false pride in one's own qualities or accomplishments, exhibited by many people who are not clinically delusional. Many people have extreme confidence in their own ability and do not know enough to understand their limitations.

For example, a number of people believe that they are among a chosen few that will be swept up into heaven by God at the end of the world, usually projected to occur sometime next week. This leads some of them to think that they are somehow better than those who, by their determination, are not thus going to be swept up.

Another example is those people who believe that the ideas bouncing around in their skulls are the greatest ones since sliced bread, when in reality they are a colossal load of codswallop. A large number of pseudoscience promoters fall into this category. It is also a virtual prerequisite for any dictator worth his salt.